I think part of it is that we are leaving the tank meta. Vayne is squishy and has to be in autoattack range to contribute to a fight, so she can easily get screwed over by CC and hard-initiation (which tanks provided a lot of). In addition, while her Silver Bolts would deal significant damage to tanks, they would still survive quite a while since they would stack armor.
Now that bruisers and assassins are coming back, she doesn't have to worry quite as much about getting locked down by CC or having the entire enemy team jumping on her face. Assassins and bruisers will typically dive her solo, so she can take advantage of her dueling capabilities and kite them out long enough to kill them before rejoining the fight.
The bot lane meta has changed quite a bit as well, but I forget the specifics. I believe that poke supports (i.e. Nami, Sona, Lulu) have become less popular and peeling supports (i.e. Braum, Janna) and engage supports (i.e. Leona, Thresh) became more popular. In addition, lane bullies such as Draven, Graves, Corki, and Lucian aren't as common as they previously were.
Trundle's in a similar position if I recall correctly; he works quite well against tanks, but he is vulnerable to CC (in this case, being kited) so too many tanks on the enemy team can screw him over. He's seems to be much better against a lone tank since his ultimate is single-target.