Actually. Didn't Byakuya hold up better against Ichigo's hollow then Hiyori did. Given that Byakuya was a shinigami and Hiyori is a Vaizer, it would seem logical that Vaizer's aren't necassarily stronger.
Also if you notice Bleach isn't that consistent with power levels. When they fought the first Menos Grande, Rukia commented how no Shinigami could fight one alone, and now we learn that captains can beat that kind with no real effort.
Lastly Isshin was capably of taking out an arrancer (Even if not fully developed) with one attack from his zanpaktou, and as far as we know he wasn't even using Shikai.
Thus I would agree that it varies from shinigami to shinigami. Zaraki in bankai for example would probably be able to beat up any espada without much difficulty. Also Soi Fon has the abillity to kill any opponent. Especially if they don't know how her abillity works. On the other hand Ukitake could have trouble fighting even a gillian depending on his state at the moment. Keep in mind that its been stated that some of the captains aren't as vocal about their abillities as others.