I resized a Vista partition with something other than Vista itself and haven't had any problems. I think I used whatever comes on the SystemRescue CD, which it stated was Vista compatible. I did this almost a year ago.
Actually, the reason I used another tool was I didn't remember that Vista itself includes a partition resizing tool. What you MUST do is make sure whatever you use is Vista compatible. At the time I did my resizing, Partition Magic, for example, was not Vista compatible. I think the popular third-party partitioning/resizing tools are now Vista compatible, but if you have an older version, you'll have to upgrade or use something else.
I don't use LILO or Grub on this computer. I boot to VL with the VL installation CD. I'd break down my usage to about 85% VL 5.9 Standard and 15% Vista.