I am just back from another great couple of weeks in Sunny Beach/ St Vlas.
Loved every minute. Visited the usual haunts, and found a few new ones.
Only down side, is the way the centre of Sunny Beach seems to be going.
This "party square" tag seems to be being pushed. The bars/clubs are cranking up the volume, while the pub crawl
hordes seem to be growing.
Nobody goes to the middle of SB for a quiet night out, but this trend which may attract some new younger tourists for a year or two will drive even more away.
Well established restaurants and bars in the streets off the square will lose out. Families are staying away from the centre at night.
Try getting from the main road down to a few well known places like Malinas, Brothers, or Tonys, and it can be awkward enough for a couple on their own. Add a couple of kids or a buggy, and it soon seems to much effort.

I know from the people I speak to, that while not staying away from the centre at night, thet are certainly going down less.
Maybe twice in a week. St Vlas seems a more pleasant alternative.