As i have a Point of View Mobii instead of a Advent Vega, but its the same tablets, and i am not sure what applicationdownload-apps there are on the Vega, but there are some places to download apps without Android Market, such as getjar.com
, also you can download the .apk files to your PC then copy them to the tablets SDCard and install them using iFileManager (that comes with the Vega).
MoDaCo's firmware is good, and it is fairly easy to install if you have some knowledge in installing drivers and such on your computer.
Here is a step by step guide on how to do it: Step by step update guide - Android @ MoDaCo
Read it carefully before you start tho
the actual flashing aint a problem, getting drivers to work on Windows7 (and i guess Vista) is abit tricky, it needs to be installed as Android Developer Bridge
and nothing else, when you connect the tablet in USB Debugging-mode(in the second part of the guide).
If it fails to install as Android Developer Bridge
when following the guide you can try to use an windowsapplication called USBDeview
(google, first hit), to see what devices are connected to your PC and just unistall the drivers for Nvidia Tegra 2 tablet
and unplug/replug the device then windows will hopefully install it correctly, some ppl needs to do this a couple of times to get it working, no idea why.
As for the guide, it tells you to update to the latest firmware in the first part, i suggest using 1.08 of advent vega firmware. Because there are some ppl that say that they have had problems with some things on the tablet after applying the custom rom on 1.09.
If you only get the install of 1.08 working and then no luck on the drivers you can always just download the 1.09 update again and you are back with a stock Vega again.
Remember that all your personal data on the tablet will be removed when flashing!!