First of all the vinca will compete with the root system of the tree.
Therefore it is not good to plant anything within the Drip line.
Now for the water,Here is an article that will explain it fairly well.
The True Effect of Irrigation on Trees
Part 1
by Katherine Woodford McDilda
This is one of your less than typical clients. A young couple has purchased a home built in the 1940s, restored its interior themselves, but has hired you to restore the landscaping.
Upon arrival, you find that the grounds have been left untended for over fifteen years and are quite overgrown. The trees on the property include some rather unique species that may have been ten to fifteen years old when the house was built.
Since irrigation is a necessity for the plans you have in mind, these existing trees have to be considered, not only for the shade they provide, but for their root systems, which are immense. It is a known fact that all trees need a certain amount of water. The new homeowners are confident that installing an irrigation system can only improve the health of their newly acquired trees, never taking into consideration that these trees have withstood many seasons without the benefit of additional water. This is not to say that there are no benefits of a controlled H2O environment.
Trees are living organisms; they have a birth, a life, then a death. "In an earlier stage of a tree's life, irrigation can be a tremendous plus," says Marty Shaw, a consulting arborist. "At this stage, it's the leading need a tree has. In the adolescent phase, the tree still needs water. However, too much can be just as bad as too little."
"This is true for the trees as they mature to adulthood as well," says Dr. Terry Tattar, Ph.D., professor of microbiology at the University of Massachusetts in Amherst. "When planned correctly, irrigation is very good. As a tree grows, it needs more water. But, as with any living thing, eating three meals a day is good for you, eating six meals a day is bad for you."
"The root system of a tree extends far beyond the drip line of the tree," Shaw explained. Next to the trunk are the buttress roots, which are the pedestal roots that the weight of the tree sits on. These large roots start out trunk size and taper down to baseball size roots, and are absolutely critical to the survival of the tree; if something happens to those roots the tree is going to fall over. Buttress roots grow in an area called the zone of rapid taper (ZORT) and become the structural roots. The roots that are after that are the feeder roots that do not provide any structural support for the tree, but they absorb most of the water.
"A good rule of thumb when installing an irrigation system is to stay outside of the drip line of a tree," says Shaw. "If you get within four feet of the trunk of a large tree and you have to sever its roots within that distance, take the tree out."
Even if you are going to just run heavy equipment that will compact the soil, you should take the tree out. The native soil that is underneath that tree is very spongy; Shaw relates it to a piece of bread. When you squeeze that piece of bread, it becomes dense and hard. That's what happens to the soil when you run equipment over it. He suggests that you rope off areas around trees to keep equipment and foot traffic to a minimum to prevent potential damage to the trees.
Dr. Tattar agreed, "Ideally, the installation should not be anywhere under this magical thing we call the drip line, even though the roots extend further out than that. It would be a good idea to give it another third away from the drip line, because, actually, they can get plenty of water with the proper spray head; they don't have to get close to the trees."
"Often, people who install irrigation systems do not understand a tree's root system, and they will cause physical damage by trenching and laying the pipe, using the vibratory plow or Ditch Witch," Dr. Tattar explained. "Installation of an irrigation system is the first danger for the tree. Even before one drop of water has hit the soil, half of the tree's root system may be cut. A large tree has a phenomenal root system twice the height of the tree. Digging anywhere near to the trunk, you are going to cut major roots, causing major problems.?
When you change the environment of an established tree, particularly by installing an irrigation system, there are several problems to contend with. You are changing the conditions of the soil, making it "less aerobic, because you are putting a lot of water around the roots of the tree that they have not had before. Many times people irrigate everyday, resulting in waterlogged soil, another term for soil with very little oxygen in it. The result "Sure death of the roots."
The root system needs air; the tree roots are growing in a thin layer on top of the soil, but they are getting oxygen diffusing from the air above. They do not grow very deep, because the oxygen does not go very deep, in most cases no more than a few inches. Therefore, if a ton of water is suddenly poured upon them and there is poor drainage, the result can be drowning or literally suffocating the root system. "This is the second danger of installing an irrigation system around established trees," says Dr. Tattar.
Shaw added, "The key is good drainage. Especially where there is clay soil and water just sits there, you start seeing higher incidents of root pathogens like root rot. Once a tree contracts this disease, you start having structural problems where it is eating away at the part of the wood that is holding the tree up."
Microorganisms are Dr Tattar's area of expertise. "When you change the conditions of the soil around an existing tree, making it wet, you are creating an ideal situation for water molds. Water molds are organisms living in the soil that grow in the saturated anaerobic soil, which in turn, grow and attack the tree's roots. The most famous of all is Phytophthora, which is a water mold that loves wet conditions. Whenever it gets wet, it becomes extremely aggressive. This organism loves excessive irrigation; when the soil gets saturated and anaerobic, it has spores that swim like sperm cells."
This mold lives in the soil and usually does not cause problems because it is under a lot of competition from other microorganisms that are very beneficial. However, when the soil is waterlogged, then it's in its ideal growing environment and becomes aggressive. Its name in Latin translates to "killer of plants." Dr. Tattar adds, "This is the one microorganism that can really be linked to wet."
"This is the organism that started the Irish Potato Famine when it rained so much in Ireland in 1843-44. Many people starved to death, or migrated to America, because it was so wet that each time a potato crop was planted, this disease (Phytophthora) killed it. If the conditions are too wet, the tree roots just cannot grow. It shows up when we have really wet weather. The most recent outbreak is in California, with the Sudden Oak Death Syndrome. They have identified this as one of the candidate organisms."
A third problem that can be caused by the disturbance of the area around the tree roots is Armillaria. This is another microorganism that attacks the root systems of the trees. While Phytophthora is called a nibbler of the roots, first attacking the smaller roots, then going after the bigger roots, Armillaria attacks the larger roots. Armillaria can be a problem associated with irrigation first by cutting the roots, then possibly overwatering, or can be caused just by disturbing the roots.
If you install an irrigation system in a poorly drained site, it is going to cause problems. As a professional, look at the lay of the land for low spots, which is all part of the hydrology of irrigation. If you see a potential problem area, you have to create a drainage system plan. In all probability, any of these situations can be dealt with before they escalate into problems. Nevertheless, sometimes this is not possible.
