These are my thoughts, the up or down votes can determine how plausible it is.
All evidence suggests that if the first Jack was not unique amongst his kind, he was certainly rare. In fact, let's go with 'rare', since on his arrival at the Tet, it correctly predicted he was being deceitful, and challenged him on it. It probably had other 'malfunctioning/ineffective' Jacks (and possibly Vickas) on which to base its assessment. Oh, the disadvantages of being a clone..
In any case, he could not have been common, or the Tet would have abandoned that approach long ago.
So the 2nd Jack (in contrast) was part of an 'effective team', having no idea of the past, and fully believing everything Sally told him via his Vicka. Seeing a version of himself, he might have immediately suspected the Scavs had found a way to clone him, or at least take on a camouflage intended to lower his defenses.
But the first Jack had his faith in the information he was given eroded over time, he had become quite distrustful, and was questioning everything, as well as deliberately and wantonly breaking protocol etc.
It was this precursor breakdown that made him more willing to save the astronauts, or perhaps even investigate the pod rather than simply fire on it from a safe distance. Or, as Sally ordered, not to visit it at all.
Vicka on the other hand, knew of Jack's doubts. At one point he makes a suggestion and she sort of groans with a 'not that again' type of reaction.
Although she was assuring Sally they were an 'effective team', she was living in a kind of denial, trying to convince herself of the same thing she was telling Sally. Besides, it had already got to a point where they had only 2 weeks left on their 'rotation' on Earth and she was just trying to get through those two weeks, intending to have a 'long serious, talk about our relationship' (I could feel all the guys shudder as I wrote that) as soon as they made it to the Tet.
So Vicka was paranoid because of Jack and watched him more closely and obsessively than was usual for 'a regular Vicka' to watch 'a regular Jack'.