Question in the title, but here's a specific example:
Let's say I cast Semester's End with Lae'zel, Vlaakith's Champion and some other creatures/planeswalkers on the field. Will all the other creatures/planeswalkers re-enter the field with one additional counter each or will LVC's replacement effect kick in as it enters and the other perms enter with two additional?
If Lae'zel is among the permanents exiled, they will all re-enter with one additional counter.
Lae'zel's effect doesn't work on other permanents because it doesn't exist yet when those creatures enter the battlefield:
614.4. Replacement effects must exist before the appropriate event occurs—they can’t “go back in time” and change something that’s already happened.
Although some permanents have replacement effects that work on their own entering the battlefield, Lae'zel's effect isn't one of them:
614.12. Some replacement effects modify how a permanent enters the battlefield. (See rules 614.1c–d.) Such effects may come from the permanent itself if they affect only that permanent (as opposed to a general subset of permanents that includes it).
