vegtherm is not worth it at all. I have one that gets flushed by d2. I put it on to help in winter time but its pretty much useless. In the summer time when its 100 degrees out it will climb 1 degree about every 2 minutes. In the winter time it will climb 1 degree in about 8 minutes.Depends on who you talk to about the RR. Some feel it makes a big difference while others feel it makes no difference at all. The reason I like the deadhead is the conversion is simplier & less fuel flows so it is easier to keep VO temps up. I wouldn't do it but that's just my opinion.
As far as a tank goes see if you can find a place locally that will make one. I could be wrong but I'm willing to bet that you can get one made much cheaper.
The only way I would use a vegtherm is if D2 & VO flow through it. The D2 would keep it flushed out. It's just not necessary though.