Just incase there are other people that are in the situation I was in, I thought i'd post how I managed to solve my problem.
I never managed to get the hang of static routes or the subsequent attempts at using NAT via osx. (Something I will dive into at a later date I hope, as it's pretty interesting). But I did managed to solve my problem with wifi range and remote-play lag.
Using an old router (WRT-54g v2) coupled with DD-WRT firmware, I configured the router into an AP (Access Point). This allowed me to share the primary router's subnet to both devices. I set each device its own static ip, as in the beginning I was experiencing a lot of IP clashes.
Once I could connect to the internet on the Vita reliably, I deleted the Vita from the "registered devices" list on the ps3. Before re-registering, i'd made sure that it was connected to the AP upstairs.
After I was able to start Remote play on the PS3 using the 'Connect via Internet' option (on the Vita). With full network signal (and minimal lag), I can now enjoy Shadow of the Colossos, ICO and Tokyo Jungle without having to disturb anyone downstairs :D
I guess the steps where:
1> Find a compatible router for use with DD-WRT custom firmware. Read up on what firmware you need and how to install it by going through the Peacock Thread: http://www.dd-wrt.com/phpBB2/viewtopic.php?t=51486 .
2> Configure your router to suit your needs. Again most of the information I found in the aforementioned thread. This helped too: http://www.dd-wrt.com/wiki/index.php/Repeating_Mode_Comparisons
3> Set static ip's for your ps3 and PS-vita.
4> (If already registered with PS3) Delete registered Vita on the PS3's 'Remote Play' list.
5> Re-registered while connected to new AP, Client Bridge, Repeater Bridge, etc.
6> Start up 'Remote Play' and press 'Connect Via Internet'.
7> Cross Fingers..
Anyways I hope this helps anyone struggling with crappy ISP provided routers and 'Lead Lined' walls such as myself..
Soba.