My question is - I have run my striping run with a keg and reflex system received 152 proof clear as water drank it many times before but now I'm thinking about doing a final run, second run, to clean it up even more. I know I'm suppose to dilute it down to 50% but will the vapors get up the column if the keg is only half full because there won't be enough water in the boiler. Also second question, should I use the banking soda and how much per liter.

"Its all relative...by that I mean, heat is pressure and as you heat up or cook your contents the vapor that it gives off will creat an internal pressure and push the vapor up the column. At this point in your process ABV means nothing, heat pushes the vapor up. Think of it as stacking --layer upon layer.
As the liquid heats it makes a layer which moves up --cause heat rises and as it moves its replaced with another layer if you are heating sufficenlty and so on. IF your heat is right its kinda like the energizer bunny-----it just keeps going and going and going. Whooweee hope all that helps."