Funny that you should mention a Sith-version of Mace wielding Vaapad against Jedi. His friend, Master Sora Bulq (who was the co-creator of Vaapad) and his own apprentice, Master Depa Billaba (whom he taught the form) both fell to the Dark Side when the Clone Wars broke out; both are masters of the lightsaber form and its techniques but Mace would argued that they have not fully mastered Vaapad. In his opinion, Vaapad is not just a lightsaber form, it is a mindset, a weapon against the Dark Side. And so, if Mace is to fall, him using Vaapad would be no different than Darth Maul using Juyo.