"She does not act as well as you."
Translation:Ŝi ne agas tiel bone kiel vi.
Precisely. Tiel means like or so (literally "in that way", "in that manner"). Kiel means "in what way", "in what manner", "how". I guess you can think of it as " She does not act in the same manner in which you act." The "which" in there is a sort of interrogative. It still seems funny though.
No, the second "kiel" is not interrogative but relative.
Esperanto shares with several other European languages the "bad habit" of re-using its interrogative pronouns as relative pronouns: "Kiu venis?" uses the same word "kiu" as "Mi konas la homon kiu venis."
So the "kiel vi" is "as you, like you".
Compare: "Mi estas same alta kiel vi", I am the same height as you; "Mi lernas kiel vi", I learn the same way as you.
Just noticed your question. I am a little surprised that no one has answered it yet. The Esperanto verb "agi" means "to carry out an action". Acting on the stage or in a film/movie would be "aktori" so if you had to translate the English sentence into Esperanto, you could say, "Ŝi ne aktoras tiel bone kiel vi."
OK so I found out it makes grammatical sense but the meaning would change slightly. Instead of "not as good as you" it would be "not better than you". It leaves ambiguity for that person to possibly be "as good as you" if you rephrase it. Sorry - wound up answering my own question.
That would be 'she does not act better than you' which allows for her to act equally as well as you. The original sentence does not. Compare 'x is not greater than y' and 'x is less than y'. If the latter is true, so is the former, however if the former is true the latter may be false (if x=y only the 'not greater than' is true). Hope this helps