He is pretending not to notice so he doesn't have to stamp you out like you are a bloodfiend.

How did everyone not know the two guys in Bosum Buddies were guys?

Mechanically? Rivenspire takes place before bangkorai, which was where a player could become a vampire. When the quests were being written, there was no scaling and no one tamriel. This means a player had a hell of a time becoming a vampire before Bangkorai because all the npcs part of the quest were 35-40 and while some low level players could manage it (i did it on a lvl 9 sorc), by and large most could not. It was also probably written before cadwells silver and gold were a thing. What this means is that when the writers were making the quest, no player could reasonably be assumes to be a vampire.

Story-wise? Maybe Verandis doesnt want to "out" the player. Maybe he doesnt want to acknowledge that the player is a vampire because it might blow that vampires cover. He doesnt know the player, where their loyalties lay (molag bal? Clavicus vile? Lamae?) And would rather not complicate the relationship.

Verandis IS an idiot though, or a traitor. The entire rivenspire runs contrary to the fighters guild quest. "Oh no! A corrupt super weapon that has the power to unleash a plague of undeath across the world! Better give it to the King of *** who is currently invading the world. Great idea! I am a genius!"

Technically you are not presented with the opportunity to become a vampire until 3 zones later, in Bangkorai. The fact that you have gotten it sooner (or that you are able to) is irrelevant to the story itself.

The situation in Rivenspire has been going on for maybe three weeks, tops, by the time you get there. His research into the Doomcrag is spotty at best, his various journals have half-translations of the Ayleidoon terms along with educated guesses as to what they mean. That's why you go to the ruins that Llereya tries to trap you both in, to get more information. Remember, the situation in Rivenspire is because he touched the relic without knowing it'd corrupt the whole thing. Before then, it was just a big magic rock, essentially.

Verandis has what is essentially a hotline to Molag Bal, who is the Lord of Domination. Bal probably jumped at the opportunity to, in his eyes, make someone who had been trying to ignore him for so long basically get down on his hands and knees and beg for help. With the very limited time he had, it was the most reliable course of action. Also remember you could totally not be in the Fighter's Guild at that point. And Tamrith is a Divines fangirl, she wouldn't give a toot about consorting with Daedra. She barely tolerates Verandis, as is.

Also, he does try to stop Tamrith and ... the other guy's... forces from proceeding further up the tower. He even mentions how dumb of an idea it was when you get about halfway there, and how stupid he was to get that far without realizing it. They press on anyway, and all the mooks get turned into bloodfiends, and Verandis is like "I TOLD YOU THIS WAS A BAD IDEA".

He appeals to whatever's left of Montclair's humanity because the two have been friends for years and he feels culpable, given that he did, albeit unintentionally, cause the whole mess. If your friend had turned into a raving murderer by your hand, would you not hesitate a bit?