It's true, btcrecover have single core bootleneck[1]. But instance on vast.ai usually use high-end CPU so it's less likely you hit the bottleneck if you only use 1 GPU. The documentation itself only mention bottleneck on multi-GPU setup[2]
I do not know how well it works, but if you rent on vast.ai and you plan to heavily use CPU, see if you rent all the cores or only some of them.
Sometimes machines with several GPUs are listen more than twice - for example server with 4 GPUs may be offered as a 4GPU and then all the cores or 4 x 1 GPU with 1/4 number of cores. And then performance may be lower, comparing to dedicated machine with 1 GPU installed only.
I just checked vast.ai again and it's true there are instance where only few cores available. However, BIP39 12 Word Seed have big speed boost if you use GPU[1] and most instance i've seen offer at least 4 cores. It shouldn't be big concern unless you wish to use both CPU and GPU with some testing to reduce search time as much as possible.
[1] https://btcrecover.readthedocs.io/en/latest/GPU_Acceleration/#performance-notes
