This is really, really, REALLY hard. Vegeta has the advantage in ranged attacks, and in close combat. But Meta Knight has mid range covered, because his sword gives him extended melee range. Meta Knight also has teleportation, and can heal himself. And if Vegeta takes that sword, Vegeta screws himself over. On a similar note, if Meta Knight loses the sword, Vegeta will wreck him. Then again, the sword can fight for itself. All in all, I give the incredibly slight edge to Meta Knight, simply because he can heal himself, and still has a few manipulations that can turn the tide.
And before anyone says anything about "combat ability," no. They both have more than enough experience in it to render it moot. By far. Vegeta has formal training, been in wars, and has done all the DBZ stuff, and Meta Knight fought in a war that lasted thousands of years, and still does stuff with Kirby and co.