As another small-batch brewer, I would say the key factors are the age of the vials and the size of the batch. If it's really 2.5 gallons, and the yeast is fresh, things should be ok. If it's more like 3 or 3.5 gallons, and the yeast has been in your fridge... then make a starter.
That said, these days I always make a starter. If I am impatient or on a tight schedule, I use dry yeast. My brew days are precious to me, and I can handle failure but would just as soon dial in as much success as is possible.
I haven't done this in a while, but I'm going to do a back-to-back stout on the yeast cake for the one I'm brewing tomorrow--assuming I like the first batch, that is!