Thinking about buying vbseo, few questions

I am thinking of purchasing a vbseo license and I have a few questions regarding it:

My forum was converted from phpbb 2.x to vBulletin and at that time a 301 redirect was setup to redirect people visiting phpbb threads to the vbulletin threads. Even though it was done almost a year back, we still get few visitors through those links. So if we were to change to rewrite setting of vBSEO, will we be able to still maintain the old redirect as well as the new one, without causing excessive server load?

Is photoplog pro compatible with vbseo and can it take benefit of url rewrite and custom meta and keyword description?

My forum is couple of years old and we get decent traffic from search engines, once we switch to vbseo and url rewrite, can we expect a sudden drop in traffic and how long would it take for us to bounce back?

All showthread.php?X=abc threads will direct to the new one automatically. So, if you have a phpbb link pointing to show thread, you don't have to do anything. Load shouldn't be noticable. It's not like 90% of your traffic comes in on those links i would imagine.

All mods/add ons are compatible with vbseo. However, this doesn't mean vbseo will do something about them in terms of SEO'ing them or re-writing them. You can set up custom rewrite rules, but generally most large scripts like gallery addons are probably too poorly designed in terms of link structure to get a solid re-write in place. I know i ran into this on photopost.

No one can say how long... it's entirely up to google. For me, I had about 2.5 weeks of 50% traffic, then boomed 300%