Which Zerker to Lvl Up First.

I don't have Vlad or Ibaraki, but have all the other servants you listed. For the main quests, you don't need neither if you can raise the others. Merlin is the top priority especially his third skill. Then Ozy or Musashi. If Solomon is an issue, just borrow a max bond herc or Cu alter.

I haven't used Vlad much, but I suspect Ibaraki works better for you as she pairs with Merlin well. Vlad really shines in an arts team but you have neither Tama nor Waver.

Emiya is not worth grailing unless you really like him as a character. The five-star archers are awesome and it is worth saving for Gil or Ishtar. Tesla is also amazing after the future strengthening. They completely overshadow Emiya.

Considering you have Merlin and lack any Arts type support, Ibaraki seems to be the better choice IMO.

Vlad would be okay too since he has 2 Buster cards but his NP won't benefit from Merlin's Buster buff.

As for grailing EMIYA, that would be up to you. The extra ATK and HP doesn't hurt and it would only take 2 grails to reach lvl90 so it's not a bad choice. He's a pretty solid servant once you've done his interludes.

Level Banana first.

Grail for love, not for power.