This question could only be answered by taking a careful look at your insurance policy or LTD plan. The rules for these things are set out in the plan and are different from case to case.
Whether it is permissible or not, my recommendation would be that a person never try to take a vacation while enrolled in a return to work plan. Even if you had a right to do it, it creates a very bad look. I have learned that disability claims are often more about “perception”, than reality. There are many negative biases and stereotypes against a person who has claimed or been denied disability benefits. It is critical to avoid any actions that feed into those negative stereotypes.
Unfortunately, I think pressing for a vacation during a return to work program would feed into these negative stereotypes (even thought that may be unfair). It would be seen by others to mean the person was not committed to returning to work (even if that is actually not the case). Perception is reality when others can determine your fate.
If your return to work proved to be unsuccessful, then the fact of the vacation would make it very hard to win the case on appeal or via the courts because of the bad perception it creates. I would urge you not to fight for this vacation because if your return to work was unsuccessful, this would prejudice your claim on appeal.
I would have no issue with a vacation if you were simply receiving LTD payments and not in a return to work program.
Please do not take my comments as a criticism of you. I have no doubt you are seeking the vacation in good faith and are taking your return to work seriously. It’s the perception, not the reality.