Can I equip items to Dauntless Vanguard when playing it from hand to attack a battlefield where the opponent is already present, if those equips are already active at my base from a previous turn?

Ruling: No, you cannot equip items to Dauntless Vanguard when playing it to a battlefield with an opponent present. Equip can only be activated at base speed, and playing a unit to a battlefield where an opponent is present opens combat, during which base speed actions cannot be performed.

Nuances:

• This is the same reason you cannot play Vanguard to a battlefield and then move another unit there from base using a standard move
• The restriction applies because there is a showdown or combat occurring, which prevents base speed game actions