Hello. Today I tried to attack with my Faceless Haven while I only had 3 snow-covered Plains and the Haven and my opponent controlled an Archon of Absolution. Was I able to pay the attack cost using the Faceless Haven and attacking with it? I figured I could because it has vigilance. Thank you.
Yes, this is in fact allowed. Since Faceless Haven had vigilance, it didn't tap to attack (C.R. 702.20b; review C.R. 508.1f), and the game lets you activate mana abilities before you pay costs to attack (C.R. 508.1i), so that you can now tap Faceless Haven for 1 and pay that cost to attack.
EDIT (May 16): Edited.