If grouting must be done “months” after stressing there are a few steps that should be taken, assuming this is allowed by project specifications. The tendon should be capped and sealed as much as possible. Corrosion inhibitor should be used as the presentation mentioned. Use of a vacuum is something I have never considered. Chances are you would never be able to seal the tendon well enough to keep a vacuum in the tendon. Very rarely can we get the tendons sealed that well. Plus, with corrugated duct, we have seen that water gets caught in the duct and is very difficult to get all of the water out. The best solution is to not allow moisture inside.