Onboard V'GER's vessel, Decker, Kirk, Spock & Bones are spinning theories about what V'GER needs to evolve. Meanwhile, Decker & The Ilia Probe are exchanging curious glances at each other, like ...
".... whuh? Did you want something?"
".... huh? whuh? What's the matter?"
The implication seems to be that all of this Carbon Unit speak is above the probe and V'GER's head, which I kind of find hard to believe. Excluding V'GER from the debate over what it needs is an interesting choice, but she's right there ... and it does seem awkward, that she doesn't pipe up and say what's on her mind on her home turf.
Then Bones asks, "this machine actually wants to join with a Human? Is that possible?" And Decker's immediate response is, "let's find out," which just seems to be there just to cut the debate short, instead of presenting the best course of action. And as Decker proceeds to make ready for the joining, Bones blurts out, "Decker! You don't know what that'll do to you!" And Decker goes, "... yes, I do, Doctor!"
No, you don't! You, yourself, just said, "... let's find out." And whilst it's interesting how Decker compares Kirk's commandeering the Enterprise to his joining with V'GER, I didn't get the impression that he was still bugged by it. He seems more interested in bringing our story full circle, with that line of dialogue, because it wouldn't be at all obvious, without it.
Bones is counting down til Earth's destruction, but it's just taken as an FYI. Kirk, Spock & Bones are debating more with eachother than with Decker. The only real pressure to do this thing is coming from The Ilia Probe and the implication most certainly seems to be that a woman as hot as Persis Khambatta was is worth dying for. It may be the least important of all possible reasons to join with V'GER but it is the only one that's seemingly exerting any force, here.
But when Ilia got fried, Decker's only reaction was to snap a belated comeback to Kirk about how to define "unwarranted." This so undermined Ilia as a motivation and yet, 'they' kept playing it up, in this movie. It's hard to say, really, but had Wise directed the actors better, at least the Ilia angle would've played. The only time the Decker/Ilia 'romance' has any heart to it is when she catches him in the hall and asks why he didn't say 'good-bye' on Delta IV. Which lasted for only a millisecond ...