Don't buy a two part vice as your only vice. Thy are not every day vices. You would only really use it for special jobs.
A standard machinist vice is what you want. Hence why the are "standard".
I'm not doubting that what you say is correct but why do you think that is the case?
Once you have trammed in the fixed end, at say a sensible distance along one half the length of the table, isn't it just a case of loosing off the nuts on the adjustable half and sliding it closer or further away to accommodate the size of work piece??
Apart from having to use parallels/something to raise the work piece off the table a bit, I can't think of another downside to this style of vice??