Learn Romanian: — Vii diseară? — Păi, nu știu încă. - Are you coming tonight?

The verb a veni (to come) has the root vi- for the "you" form. The second -i is the standard ending for "tu" in the present tense. So, vi + i = vii. If you see vi with only one i, it is a completely different word: the shortened pronoun for "to you guys" (as in vi le dau - I give them to you).

Is there a difference between diseară and astă-seară for "tonight"?

They mean exactly the same thing and are used interchangeably. Diseară is a contraction of de + seară (of the evening), while astă-seară comes from această + seară (this evening). Diseară is slightly more common in casual spoken Romanian.

How exactly is the word păi used here? Can I use it anytime I would say "well" in English?

Păi is a very common conversational filler, much like "well" in English, used when hesitating, softening a refusal, or starting an explanation. However, it is strictly informal. You would not use păi in formal writing, whereas the English "well" is more versatile.

Încă is translated as "yet" here, but does it not usually mean "still"?