Andorians tend to be warlike, so if they had detected a warp signature, it's unlikely they would have sent only a single science vessel to investigate.
Vulcans claimed (or so Troi told the story) that they were just "passing through" when they detected the signal. But nobody passes through star systems - there's no point in that, unless the system offers something of interest. It's astronomically unlikely that a shortcut from A to B would involve Sol in any fashion, say, even in a galaxy as densely populated as the Trek one.
So there was something at Sol that interested Vulcans. Earth, perhaps, as we learn in ENT that they spied on our progress and lack thereof until the 20th century at least. But Earth did not appear to interest Andorians in the slightest, so there's asymmetry right there already.
Or then something else interested the Vulcans. For all we know, their conflict with the Andorians included a sideshow at Sol!
So, possibly an Andorian ship detecting a warp signature would not even bother to report it. Possibly it would immediately send over a party to sign a military alliance with a virgin world so conveniently located right next door to Vulcan. I doubt there would be Andorian resources to spare for a conquest or even a sterilization, though.
Timo Saloniemi