Good questions! It should be fine to set max_queue_size to 20 and higher values, however we have not done enough load testing yet on realistic scenarios, especially with larger subnet sizes. So we are proposing some conservative value here with the intention of increasing it later on.
Regarding pre_signatures_to_create_in_advance, this parameters does not affect vetkd, as the scheme does not use presginatures, differently than ECDSA and Schnorr. However since vetkeys reuses the same chain key config record as ECDSA and Schnorr, this value has to be set currently. Note that until recently it was not possible to set it to 0, so 1 was the minimum, see here for more details: feat: CRP-2618 Allow zero `pre_signatures_to_create_in_advance` in vetKD `ChainKeyConfig` by eichhorl · Pull Request #5014 · dfinity/ic · GitHub