Steadfast Armasaur

• WOTC Official Reminder Textofficial rules
  Vigilance — Attacking doesn't cause this creature to tap.

• WWizards of the Coast2017-09-29
  Official Ruling
  Tapping an attacking or blocking creature doesn't remove it from combat. If the target of Steadfast Armasaur's ability survives the damage, Steadfast Armasaur will deal combat damage to and be dealt combat damage by that creature as normal.
• WWizards of the Coast2017-09-29
  Official Ruling
  If Steadfast Armasaur is no longer on the battlefield as its ability resolves, use its toughness as it last existed on the battlefield to determine how damage is dealt.

Rulings via Scryfall