I thought I would have problems with a boy, a girl was easy - skirts and dresses to show off the nappies
but actually it has been fine. Ben is chubby and short but most brands fit him. I like dungarees alot as they are always roomy in the bum. He has jeans from a few places, George, Cherokee, a couple of Next ones, although these are the smallest and he has just about grown out of his 12-18m Next jeans at 16 months, so I would avoid Next really.
My favourite way to avoid problems is to buy crops (maybe pick some up in the summer clothes sales) in the size bigger, especially if they have an adjustable waist.
He is still wearing a couple of pairs of joggers from Mini Mode which are 9-12m (they are nice and stretchy around the bum and he is short). I have a couple of Cut 4 Cloth trousers but as you say they are pricey and I keep them for best. They are great though.
Oh and for vests of course you can get vest extenders which make them last better.