When I (or my kisd and/or wife) eat at a friend's or aquaintances house, we just make sure that 1) they know that we are vegan 2) they know what that means 3) that they have "positive" ideas of what we can eat (ie, instead of saying what we don't eat, we tell them what we like, down to the brand and where the item is available), and most importantly, we 4) bring something to share that can feed the whole family and some curious guests, just in case. Often, but not always, we also 5) eat ahead of time, so that if the food situation turns bad, we're not starving and grumpy.
This plan has helped a lot. Especially in cases like my son being invited to his friend's B-day party, and the family only serving non-vegan cake, and non-vegan cow milk ice cream, and celebrating with a pinata full of non-vegan candies. In that case, we brought our own candy, our own vegan soy ice cream, and we promised out kid cake for later.
I find it is a lot less rude to really be clear about my eating habits ASAP, so that the person who does the inviting can really think and process what that means, and be prepared for it. I usually take charge and just tell them to cook me barilla/spartan/etc. brand pasta, with simple tomato sauce from a can (i specify the brand), if they insist on cooking for us. What's most difficult is when people really try but get something wrong (as in buying non-vegan but vegetarian boca burgers instead of the vegan ones).
This has worked even when we visit family abroad, who have absolutely no concept of veganism. We don't count on them to figure it out, instead, we do the research and we suggest what we can eat. Only after 12 years of dealing with me (and 8 years of my brother, and 6 years of my wife, and 3 of our kids), they've actually gotten a good drift of it, and now offer vegan foods, and they actually get into it and offer some of the best native veggies ever. The native veggies to Chile are AMAZING. I have a better meal than most people since I get to try those amazing treats.
I love to travel in unfamiliar areas, and that's the only time when being vegan takes more than a second thought.
What we've learned to do is to plan the day around a meal (and now with kids, nap time) instead of planning the day around an activity. That means that we pick where and when we will eat before we figure out where will will go and what we will do.
The best results have been from going online and using the various services to find vegan restaurants where ever it is that we are. Then, we plan the day around that restaurant. The best experiences we've had have been that way.
The next best thing, when we don't know where will be stoping, is to eat at restaurants that most often have vegan foods. That means we have a backup of eating mexican, or italian, or our perenial favorite, chinese. Those restaurants most often have a dish which is vegan or at least can be easily and simply modified to be vegan without much of a hassle. Examles are the aglio olio sans parmesan at Italian restaurants, or veggies and tofu at chinese restaurants, or a simple bean and rice burrito sans meat and cheese at a mexican restaurant. As a final backup, if forced, we will eat at Subway, but luckily, there are often other cahin options including Qdoba, Bruegger's Bagels, Evo's, or one of many wrap places that've popped up.
It's been pretty simple, even if all the details make it seem otherwise. We just stick to our routine, and it's all panned out without a problem for many many years.