On the other hand, they might just wring out the best response possible at a slightly greater risk. Here I'm assuming your parents shouldn't try a PRC vaccine, so it's virus vector Russian or mMRA Pfizer, both essentially accomplishing the same thing, both with high overall efficacy.
Or look at it this way: if they don't get a good result from either of these two vaccines, they're not likely to fair well from a real infection. Maybe. We really don't known why even at their age most people survive.
The other factor is how much you trust quality control. Pfizer > Russia >> PRC in my book, but I have absolutely no read on Russian quality. But also note Pfizer/BioNTech is useless if not properly handled to its strict temp and physical requirements.
Just as an example Sinovac is not even used in old people in China as far as I know.
There’s a similar question with AstraZeneca right now, German vaccination committee would like to see more Phase 3 data on old people before moving into Phase 4.