I cannot comment on the VPN issues with linksys routers but I can comment on using DD-WRT. One of th things it does way better than the default firmware is provide a much smoother up/downlad speed for simultaneous communications. The stock firmware is very choppy when it comes to simultaneous throughput. This is true for the WRT54G/GL/GS models. I run it on my home router (now just an access point) and I have a client that would not spring for anyuthing more than a WRT54G and it makes things tolerable for the 12 people behind it. It also stopped the router crashes.
DD-WRT may help things out.