I wouldn’t think that vibration would be an issue, no.

But I would think the electrical field generated by the power components of each device might affect the WiFi capabilities of each. I’d recommend separating them by at least half a meter. That could be in any direction. Or, if it works for now, you could leave it on top, with the knowledge that at some point, you may get some wifi interference .

Are you using the Port just to give you a line in? You don't use the Port to feed a HiFi? I am guessing it is just a line in. 

If so, how did this come about? A Sonos Five would be so much better than a Port and a One.

Perhaps I have misinterpreted the situation. 

Apart from two wireless transceivers in close proximity potentially deafening one another, I’d be concerned about heat. The Port cools itself by conduction through the top surface, and then by convection/radiation into the room.