I used to think I needed the supplements, after I was diagnosed as having low vitamin D counts. So the doctor recommended some ridiculously high amounts and I took that for several months, but when they re-tested me, nothing had changed. Also, I live in Southern California and I’m pretty sure I get enough sun-light, in fact because I burn so easily, I try to make sure not to get too much sun. I think my vitamin D situation is hereditary, and seems not to be effected by taking (or not taking) the supplements, plus I drink milk and eat dairy, which contains vitamin D.
On the other hand, some people, especially when they get older, may not be able to absorb B-12 as easily from food, and need to take supplements or get injections. Most vitamins are best absorbed by eating foods that contain those ingredients, but with B-12 (especially in older people) that might not be the case, but that goes for folks who eat meat as well as vegetatians.
So IMO, you probably don’t need supplements, but you need to make darned sure that you are eating enough of the right foods. I, for instance, eat ground flax meal every single day, because one of the nutrients that our bodies need is Omega 3 fatty acids. Most people get that from fish, but the best source for vegetarians is ground flax meal (it has to be ground, rather than whole, because the whole seeds are not digested and simply pass on through).
Here is some Information regarding what nutrients vegetarians need and which foods contain these nutrients.
Here is some info about getting b-12 from Vegan Sources (if you don’t eat eggs or dairy).