1)She dive bombed Vhagar from higher as she is faster and big enough to fatally injure vhagar.

2)Attacking/torn her wings from her back. Meleys is said to be the fastest dragon and she also looks pretty nimble. Vhagar would probably not be flexible enough to reach meleys and if she breath fire, meleys could have repeated the tactic until vhagar exhaustion.

I know the story has not be written this way, but i think all dragons up to syrax size could have defeated vhagar if they fought wisely and used these strategies. Sure, meleys and vermithor had the higher chance to kill her. But being bigger is not an autowin in a sky fight.

I just want this post to make you think that yes vhagar has a physical advantage, but these tactics could have worked if GRRM didnt wanted his story to be a horrible tragedy or if rhaenys fought more wisely. Also, some people have to acknowledge that vhagar has some physical disadvantage, shes slower and not so agile!

PS: Yes i know vhagar is older and has experiences, but it doesnt make her unkilable