Breakfast is 'Snice

In my experience, the trickiest meal for a vegan is breakfast, while travelling or not. I have spent many a morning thankful I packed peanut butter to go along with my toast and orange juice and hope it would keep me full until lunch. While I whole-heartedly recommend packing a little jar of peanut butter for your travels (or another nut butter of your choice), I also make every effort to seek out vegan breakfasts wherever I am.
Both times I was in New York, I was fortunate enough to be staying within walking distance of 'Snice, a cafe that catered to the latte-and-laptop crowd that happened to have delicious vegan options for all three meals. I hear it has closed, but it made all the difference for my stays in NYC. There is something about starting your day with actual options in your breakfast menu, and setting off with warm food in your belly.
If there isn't such a lovely vegan-friendly cafe near wherever you are staying, it helps if you have use of a fridge. Find a market or grocery store and pick up a few small things that make your morning easier and your stomach happy, like bread with hummus and olives, and some fruit. It is not only cheaper than whatever you could get in a restaurant, it will make your entire day just a little bit more fulfilled. And if you do find a place with an amazing vegan breakfast, be sure to go at least once, even it costs a little bit more than your market finds, and profusely thank them for helping out a travelling vegan.