I don't know the ins and outs of WV politics, unsurprisingly. But if Sen. Manchin didn't believe his vote to confirm AG Sessions would result in his being "primaried", he's not smart enough to be where he is. He knows that will happen. The question, IMO, is whether he will try to stick it through as a D, flip in the next year or so to R, or retire.
Running as a D would cost him a lot of $$ in the primary, and he would not receive much support from the national level D party and its big-$$ contributors if he won. Flipping to R is also risky, as the WV D Party people would be highly motivated to defeat him, and his welcome by WV and many national level Rs would be uncertain.
That's how it looks from this side of the continent.