Once you’ve activated the ability granted by Frondland Felidar, causing the creature to lose that ability (such as by removing Frondland Felidar from the battlefield) won’t affect the ability on the stack.

Frondland Felidar’s last ability applies to itself as long as it still has vigilance.

You can activate the ability granted by Frondland Felidar after attacking but before blockers are chosen.

Tapping an attacking creature doesn’t remove it from combat.