Depends on the time of year, and the extent to which you feel the cold.

Towards the end of September you might need a thin jacket. In October a thermal vest could be a good idea, it was darned cold one year in St Konstantin, we needed an electric radiator in the room.

Peter

Up until September I would say neither, from then onwards a thin jumper or very light jacket. Never needed either last year, 1st 2 weeks in September.

Toonman

Wouldnt worry too much.

we were out in Sunny Beach a few nights last September, and if it was chilly, the stall holders soon had every shape, size ,style and colour of jacket and jumper on sale.
It was a laugh sitting in the pub seeing the odd mixtures of styles that came in. Strappy sandals, with woolly jumpers all round.

10pm here tonight and its 27c outside....rarely gets cold enough for a jumper etc until late september