Klingons are presented as equals in TOS, but dumber than humans, and easily outwitted. Twice they declare a truce. In Elaan of Troyius, they are quickly beaten, and run away. In ST3, they are outwitted and beaten.
The Romulans are always outwitted and/or beaten. TNG makes it worse by adding "mustache twirling" to their attributes. Humans are just smarter, I guess. Even Q is jealous.
Voyager encounters the "Borg collective" 4 main times. In scorpion/the gift, they are saved by others, like Kes. In other appearances, they narrowly escape, or get help. It's only in the finale that they are really beaten. Voyager also has an important reason to explore borg storylines:
They have a borg main character.
It's no different on TNG and DS9, with all those many "Klingon episodes" where Worf is somehow constantly getting involved, and having influence in the highest rungs of power in Klingon society. He regularly discusses politics with the klingon president, even though "Worf is a pariah" is his main attribute amongst them(Or "Quark is a Pariah," yet also is constantly involved with Ferengi power. I just realized that. Worf must be a Klingon Prince, and Quark a Ferengi prince).
The Vidiians were never going to be seen again by the time of Think Tank(unless by time travel. I don't see how an off hand remark by the Costanza guy would change anything.
Besides, everything else he told Janeway was a lie. He pretended to be surprised by learning of/meeting Seven, but he actually had acquired Voyager's database and fabricated an elaborate scenario to specifically capture her. The think tank had a bad reputation locally and probably never saw a Vidiian. He's a big league con artist.