Cape Cod (no e) is lovely at the end of June.
I would stay somewhere where you have a kitchen because vegan restaurant choices are few, though there are some possibilities in Provincetown, including Karoo Kafe http://www.karookafe.com/

You could stay in Ptown itself or in North Truro for ease of access. Wellfleet is lovely too.
It is getting to be time to reserve for June.

Ptown.

Eastham, Wellfleet, Truro, N. Truro or Provincetown. There are lots of places to eat and my vegetarian DIL has never gone hungry.

I know Wellfleet best, and I think it's a great location for access to the National Seashore and the Cape Cod Bike Trail. Truro, Provincetown, and Eastham will work as well.

I think you will find many restaurants are vegan friendly and can accommodate you. The Juice in Wellfleet has some vegan options for lunch and dinner, and the Box Lunch (a few locations, wrap sandwiches, good for bringing on a walk or to the beach) has vegan options. Most places will have one or two things on their menu for you and there are quite a few places to eat. I think you'll be happy.

I noticed Vegetable Napoleon on the menus of both Mac's Shack and Finally JP's in Wellfleet. Also the PBBoulangerie has a couple of options for you.

Also Orleans.

PBBoulangerie I think has vegetarian not vegan options, though I could be wrong, and I think the Napolean you mention at Mac's has cheese.

But people might make food to accommodate you, especially if you are earlier in June when things are not crazy yet. Here's a link I found by googling vegan food Provincetown and it lists a few options, including Napi's which I had forgotten about in terms of vegan food.
http://www.thechubbyvegan.com/2011/07/provincetown-vegan-guide.html

But still, if you are there for more than a few days, you'll do well to have a kitchen, or at least a fridge. There are some nice farmers markets around and you could have good salads etc.

Thanks everybody.

Sorry about the typos. I thought I had pressed "preview" but obviously I pressed "submit." Hopefully this post will be free of spelling errors.

Thanks for all the suggestions; I really appreciate them and will check them all. Vegan is quite different from vegetarian; the latter is much easier to accommodate.

The reason I asked for vegan specifically is because I spent a week on Block Island a few years back. There was one place with two vegan sandwiches. They were good but got old fast. Living on salads for a week is not too appealing either. We live in CA and it seems there are more and more vegan restaurants or places that will make something vegan for us there. But that has not been the case everywhere we travel. I understand it can be difficult for restaurants to cater to my food choices.

I'm adding KarooKafe no matter where we stay. I agree, a kitchen will be a must for us.

I think I am leaning towards Wellfleet.

Jubilada, the Mac's Shack menu did not list cheese in the description, and the PBBoulangerie mentions vegetarian and vegan (in French so I may be mistranslating!) but I checked before I posted. I travel with a vegetarian so I am aware of menu options.

I agree having a kitchen would solve the problem but for four nights it's not always practical.

cw, I stand corrected. I don't trust PB so that bias may be coming through.

egnolive, my area of western ma has lots of vegan options, the cape less the seafood focus being central. But do go to Napis also, their food is very good and I think that Blackfish in Truro will accommodate you even if there are not vegan items on the main course menu if you call ahead on a weeknight.

No problem, jubilada.

Terra Luna in Truro also has one or two possibilities. Agree that calling ahead may help with menu options.

My daughter and son-in-law are vegan but if they can't find vegan restaraunts they look for Thai food, or Chinese, Vietmanese etc so they can get vegan choices. There is a lovely Thai restaurant in Hyannis but that is still an hour or so away from Provincetown.

There is a decent Thai restaurant in, I think, Eastham, also.

Thanks everyone.

Italian, and Asian food often have vegan options, but not always. You have to make sure the sauces are vegan, which is not always the case.

Calling ahead is always a good option; some restaurants are better at accommodating than others.

I've got options now. I appreciate all the input.

Karoo Kafe will be opening a restaurant in Eastham this spring.