Do ya think that the viability of the yeast decreases significantly when the lager is set at higher temps for a a “diactyl rest” while left on the yeast cake? Say like 64 F for a week? Planning on brewing a Rye bock (1.060 OG)from a 2 week old lager yeast ( WLP833) that has been sitting at 64F for 7 days.
Nope.
Lager yeast would just as soon be at 64 degrees than 50. It’s just that the beer tastes better when you ferment at 50.
I use the Brulosopher step fermentation for lagers and have had much success. Your situation may have a bit to do with how long the yeast fermented prior to the D-rest period. If it wasn’t too terribly long, then your yeast should be just fine for repitching. If it was a month or more, then the D-rest was probably late and the warm temp may have taken a week to obtain the reducing effect on diacetyl, but the slurry should still be okay, unless you fermented for something like 8 weeks in the primary - then I would rack and do a starter with the slurry (a couple-three tablespoons or so), rather than relying on the slurry going directly to a 1060 beer.
I also assume you are coming from a smaller ABV beer here.
Good luck with your rye bock! Sounds great.