At first I thought that it could have been LE confusing The Mountain Barn with the Mountain Side, as her family went to the Mountain Barn looking for her, but, now that I read the link mentioned the time I no longer think that. The "12 Oclock tells me it was a specific conversation made to L.E. in regards to The Mountain Side.
Again, being so early in the search, I am thinking that LE was heavy in the area, and were talking to many people. If Vanessas' Aunt was out looking for her at 3:30, and chased after a State Police cruiser, I think it's fair to say that the State Police were first on the case, meaning they would have had time by 7

M (3 hours plus) to talk to a lot of people.
To answer your question... 60/40. Either a family member,(60) or someone else LE talked to. A store employee that thought they may have seen her, after LE saturating the area going there randomly because they were just starting the investigation, thinking there may be cams there,wondering if the cams may have seen her jogging past. (40)
Knowing she specifically was "getting a drink, I am narrowing it down between the two.
If she was seen on Video, then I'd say 100% family, and the reason I say that is, relating to the "secret lover" scenario, if she lies to her family before she leaves the house, saying she is going to the store, then, instead , she meets someone, that means she would have had to communicate with him before she left, leaving the electronic footprint.
If she had planned it without communicating first,(somehow) then why drive to meet him, then drive back in a half hour or so after, then set back out to meet him? Why not just set out to meet him and forget about the short drive first?
If you say that she set out for the store, then met him, planned on going someplace with him, drove the car back home, then set out to meet him again, then why the lie in advance about going to the store if she didn't know she was going to meet him in the first place?