Yes.. It may take up to 15 gigs on install..Give it a lot more space than that because many softwares still only work if they install to program files in the primary live partition on C drive.. That is where you'll be putting 7 on..
Btw 7 - 64 will make program files twice..one will be program files 86 ( it will says that this is what it is )..the other will be program files 64 ( it won't say 64 )..programs will find the correct one to install into ..
Unless you want to format your HD ( if you only have the 1 HD ) install 7 next to Vista ( not over it ) ..Later recover whatever files you want from the Vista partition..
You can then either "extend" the live partition with 7 installed .. or keep the old vista space for storage ( after formatting that vista partition )..
Better still, is install 7 on it's own HD..and keep your files on at least one other HD..or more
Make sure that you have at least 4 gigs ( 8 and up is better )..of RAM for a "vanilla" 7-64 install..otherwise it may run like molasses..