There are now less than six months until support ends for Windows Vista.
Once support ends, Microsoft will post no further updates and this puts Vista into the same category as Windows XP - unsafe at any speed.
If you're still using Vista, your options are limited. Some PCs from the early Windows 7 days came with dual licenses for Vista and 7. If that is present on your PCs, you should make the upgrade as soon as possible.
If you are thinking about Windows 10 for your Vista PCs, plan on new installs, not upgrades.
Given the age of the PCs involved, new PCs with Windows 10 are probably a better bet.