If your Seven is a full version, retail or OEM doesn't matter, it's OK to use your Vista key on another computer.
If your Seven is an upgrade version, you have a moral problem. According to the EULA (End User Licence Agreement) you are entitled to use an upgrade version if you have a valid, licensed and activated copy of an earlier Windows OS. You loose your right to use this earlier licence (XP, Vista) when you activate Seven Upgrade version.
It's up to each and everyone to set up personal moral and ethical boundaries. I think a vast majority would re-use old Vista.