You wouldn't really need it to be in vector. You'd just have to separate the CMYK out. Or if it's just a few colors, you can split it into spot colors so each one has it's own layer. That's all you really need to print stuff, really.
As far as banners/posters/etc, if it's something that's to be done in small quantities, then you're more than likely going to be printing digitally anyways so the color count doesn't matter. Even still if you're doing offset printing, unless it's a 2 color job you're not going to save any money since all other jobs are just CMYK anyways (unless you use a PMS color but that's another story).
The big difference on large format stuff though is finding someone who can print digitally, which saves you a TON of money. The past 3-4 years have really helped because the digital presses have gotten larger and larger. They now have inks which can adhere to just about any surface as well, including white ink.
T-Shirts however are a whole different ballgame, and it relies entirely on what the company you're dealing with has when it comes to equipment. Usually though you want to keep it 2-3 color.