An EXPERIENCED ultra high vacuum tig welder should be more than happy to weld.
Don't mean to offend but do you kinow what you are talking about?
TIG = tungsten inert gas (TIG) welding, is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contamination by an inert shielding gas (argon or helium)
so no vacuum to be found.
For this reason alone I would question your other statements, yet the section to be welded need to be clean but to the extend that you descibe.
The following is quoted from the Lincoln electric site - USA (?) manufacturer of TIG equipment.
To overcome these challenges, operators need to follow the rules of thumb and equipment-selection guidelines offered here...
Base-metal preparation: To weld aluminum, operators must take care to clean the base material and remove any aluminum oxide and hydrocarbon contamination from oils or cutting solvents. Aluminum oxide on the surface of the material melts at 3,700 F while the base-material aluminum underneath will melt at 1,200 F. Therefore, leaving any oxide on the surface of the base material will inhibit penetration of the filler metal into the workpiece.
To remove aluminum oxides, use a stainless-steel bristle wire brush or solvents and etching solutions. When using a stainless-steel brush, brush only in one direction. Take care to not brush too roughly: rough brushing can further imbed the oxides in the work piece. Also, use the brush only on aluminum work-don't clean aluminum with a brush that's been used on stainless or carbon steel. When using chemical etching solutions, make sure to remove them from the work before welding.
To minimize the risk of hydrocarbons from oils or cutting solvents entering the weld, remove them with a degreaser. Check that the degreaser does not contain any hydrocarbons.
Your question says a lot. Thank you for clarifying what I suggested.