I'll address it, since it is an issue that I have had to deal with a ton over the years (managing 100s of people engaged in business travel).

IMHO, whether the behavior is ethical or not has to do with facts we don't know - whether the voucher is from a re-fare or CS/VDB issue.

Practically, a voucher is cash to me. I've never taken a VDB, as I'm away from home too much as is! However if someone makes a choice to trade their time for a voucher, they've earned that "cash" just as much as I've earned the FF miles I use for personal stuff all the time.

On the other hand, if they got a re-fare refund on a ticket that was expensed for the full price, that voucher is "cash" that belongs to the company.

Unethical - buying and expensing a $2000 full fare ticket, then buying and flying on a $400 non-refundable.

Unethical - getting a re-fare voucher (or cash) but expensing the original price.

Ethical - taking a VDB or getting a CS voucher for a screwup and then using those to pay for your personal travel.

It would amaze you the types of stuff that people at consultancies and large corporations try to get away with. Imagine how easy it would be for any of us to make it look like a ticket cost $1200 when it really only cost us $400. This is one of many reasons why a lot of companies only allow booking through corporate travel agencies. One bad apple....