Supermarkets tend to do multipacks - big multipacks of plain white vests and babygros, packs of 3 of patterned ones, Mothercare, Boots, M&S, John Lewis and Next also do 3 packs of babygros. For some reason the coordinating vests seem to come in 5 packs. I was always very particular about vests & babygros 'matching' although I've no idea why, just made life harder for myself!
I would say that it might be worth getting a few from different places, as the sizing is far from standard! For example, on my DD Next & M&S come up huge, Mothercare a bit smaller, Sainsbury's a bit smaller again, and forr her Asda and Tesco are true to size. She's 8 months now and the other day was wearing a 0-3 month M&S babygro with a 9-12 month Sainsbury's cardigan,, both fit her fine!
With vests they seem to come as short sleeved (standard, most are like this), sleeveless (pointless for a winter baby), and long-sleeved. In the depths of winter my DD had a long sleeved vest under a babygro, day and night. I'd save my favourites for daywear. She didn't wear outfits or tops and leggings until she was about 3 months old and still regularly wears babygros as they are so easy and comfy.