How to get WordPerfect 5.1 working under Vista?

Run it inside DOSBox. That should solve your problems.

Their emulation is quiet decent, they have solved problems with EMS/DOS4GW/whatever etc.

I think it should run any DOS application pretty much OK.

I would suggest creating a Virual Machine running DOS. You can download Virtual PC for free here. You will need a copy of DOS to do the install.