You can almost count on spending a few hours tuning up and accurizing a cheap vise if you want it to be accurate. It is also my experience that the cheap clones are "spongy feeling" when it comes to tightening them up firmly. It could be a combination of poor thrust bearing, poor screw and nut fit, compounding of clearances in keys and keyways used to locate the fixed jaw, soft low grade bolts used to hold the fixed jaw, said bolts being a bit too small to begin with.
Also, the cheap vises may have had their hardened jaws ground in place, so the top may not be parallel with the bottom. Frequently, the counterbored holes in the vise jaws are too constraining, and are out of position enough, that the jaw insert does not nest properly on the vise slide. So, the first time you take them off and put them back on, they are a bear to try to get level again.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)