. When you overclock RAM, you alleviate a bottleneck in scenarios with a high cache miss rate (such as gaming especially). This puts further stress onto the CPU. It means that your CPU undervolt wasn't truly stable.
Now for your direct question, no, a vcore undervolt will not effect memory stability itself. That is, if your memory is truly stable after overclocked/tightened, a CPU overclock or vcore undervolt won't effect it.
Best practice in my experience is to keep CPU at stock/stable settings, overclock/tighten the ram, then once happy with the ram and verified stable (with ram tests, I prefer karhu), begin the CPU over clocking.