Can Flash be legally activated if the only unit a player has is at Vilemaw's Lair, and what happens when it resolves?

Ruling: Flash can be legally activated even if the only unit is at Vilemaw's Lair. The card resolves but has no effect because units at Vilemaw's Lair cannot be moved to base in any way.

Sequence:

• Flash is activated and resources are paid
• The player selects the unit at Vilemaw's Lair as a target
• Flash resolves but the unit cannot move due to Vilemaw's Lair's restriction
• The player loses Flash and the resources spent

Nuances:

• Flash can be cast with 0 targets since it says "up to" 2 units
• Any card that says "up to X" allows zero as a valid option
• Vilemaw's Lair prevents moving in any way, not just standard move actions