You are not going to find many people running plenum systems anymore-- they were quite popular 10+ years ago. It is not that they don't work, it is just most folks found that they could get the same results using a deep, or shallow, or no sandbed at all providing they set their systems up correctly and plenums just kind of became unnecessary.
Sorry, my question is: Why would I have to use a pump to vacuum the plenum once set up.
I am going to use a plenum, I just want to know if the riser tube is necessary
It's funny - I was just reading something on this today at http://www.garf.org/trever/SHOWTR/pages/2TREV3rd.html
don't ever allow the water below the plenum to get into your tank water.
The draining process of the Jaubert method is only a recent concept(5-7yrs old)and the drain should be no more than 1/4 hose to drain the water,
I have a close friend who still uses this method with great success. When i first start my 92 bowfront - I used both Jaubert and Berlin methods - see the before and after images inline here.
HTH
House