My opponent is attacking into my battlefield with Vi, peacekeeper. If I reveal temporal breach and use it on my own unit, am I the attacker now?

No — you're still the defender, and your opponent is still the attacker. Attacker/Defender is fixed by who applied Contested to that battlefield, and that was your opponent when they moved into your battlefield with Vi, Peacekeeper (CR 466.2.c). Temporal Breach banishes your unit and replays it to the same location — a battlefield you already control — so your own unit re-entering applies no Contested, and the designation doesn't flip. Your replayed unit just rejoins as a Defender.

This is the same principle as the Rengar/Temporal Breach interaction (FAQ #11731): replaying units during combat doesn't reassign attacker/defender, and only a unit that first applies Contested makes its controller the attacker. Vi's own "when I attack" trigger already fired when she gained the Attacker designation at combat start, so nothing about your Temporal Breach changes her status either.

One note: since it's played from facedown, Temporal Breach must target a unit at the battlefield where it was hidden, so it has to hit a unit there (yours qualifies).