I think at this point in their decline Alma could have succeeded in killing Vehk. Not due to any great power of hers, but because Vivec was more or less resigned to death, whereas Alma had crazy on her side. She was consolidating all the power she had left and had the spirit to fight, Vivec not so much.
"That is very sad. I presume she killed Sotha Sil. I thought she might harm me. And I presume she tried to kill you, Nerevarine. It is all very sad."--Vivec
That was while they were gods. At the end of Morrowind/Mournhold, they are mortal.