Windows 7 is officially supported by Vocaloid 3.
For Vocaloid 2, it depends on each product. For example, Miku, Rin/Len and Luka officially support Windows 7. However, Prima does not support it officially. You can use Vocaloid 2 vocals in your Vocaloid 3 editor running on Windows 7 by importing their voicebanks to Vocaloid 3 environment. However, in such case, the vocals may sound a little bit differently than in Vocaloid 2 editors.
The original Vocaloid engine (Vocaloid 1) does not support Windows 7.
This means if you have a Vocaloid3 editor you can use all but the original Vocaloid vocals on Windows 7.
There have been reports of both engines working on the OS, but due to their unsupported status could possibly be rendered unable to function at any point in the OS updates. Furthermore, particularly with Vocaloid the engine can prove to be buggy and glitches or crashes quite a bit.