Vector will always be there for the Agent, as long as he can be. I found that the one Sha’enra’tavas wanted to share her secrets with was the one she really couldn’t, not without having those secrets shared to the Killiks and Joiners, and therein lay so much tragedy.
I’m really happy you said this, because it’s been on my mind too and I think it’s something that frequently gets glossed over in the character discussions and fanfiction. As a companion to a spy, Vector doesn’t make a whole lot of logical sense. Don’t get me wrong: I love him! He’s either my favorite or second character in the game, depending upon how recently I’ve reread Andronikos Revel’s love letters.
But I don’t really understand why Keeper would enable Vector’s assignment to Cipher Nine. It’s hard to be sneaky when your partner is mind-linked to an entire civilization. Unless I’m overestimating Vector’s long-range telepathy (which is entirely possible), no matter where Cipher Nine is or what (s)he is doing, the entire Oroboro nest knows. It seems to me it would be a huge liability, and is exactly why Velkana, at least, has kept so many secrets from Vector when what she really wants is to tell him everything (including, sadly, her name).