You've never told us the router you have. (Nonody'd asked!). Now need to know.
I would suggest doing the downloads simultaneously. I think also we've got quite a way to go to bottom this out.
When you've done the downloads with the laptops in the current position, can you swap them round and report. Even if the Vaio speed increases slightly, it'll speak to the effect of the PC that's nearest to the wireless router getting better service (because the signal is slightly stronger).
Then there's the 5100 AGN and its configuration. There's a lot of stuff about on this. Have a look at these two links and compare against your configuration. As we're focusing now on the Vaio, simultaneous tests won't be necessary for the next stages.
http://forum.notebookreview.com/show...=338409&page=2
http://forum.notebookreview.com/showthread.php?t=335716
http://forum.notebookreview.com/showthread.php?t=441742 (for debate on bluetooth kept off)
Let us know.