Hello,
If you mean use the same product key to install and use Vista on multiple machines
simultaneously then it will not work.
However, if you were to cease using Vista on your current system and install vista on a new computer using the product key from your old system (i.e. using the product key only on one machine at any one time), then this would work.
NOTE however that while it works, Windows OEM product keys (specifically Certificate of Authenticity labels) are non-transferable, and transferring product keys is not allowed under the EULA.