I was wondering, do Meeko and/or Vigilance level like all the other followers, which is to say if I encounter them at level 40 will they be stronger than if I encounter them at, say, level 20?

I really don't know. But he is strong enough. We have killed a dragon together today. He is pretty smart..

No, they will not level and in fact they will not level even after you use a wabbajack/disable-enable in console AFTER you have recruited them.
The best way to apporach this is to wait until their at the max level (25 for meeko, 50 for vil), use disable/enable, then recruit/buy them and they will have their maximum stats.

meeko's max level is 25, vililance's max level is 50, and barbas's is 48(?).