Some specifics might help in getting some help.

"How long does a vacation home have to be a rental to be considered sale of business property"

Okay, I'll try:

You have to determine if this was Vacation home (with the minimal rental per the IRS) or it was converted from personal vacation home to Rental as investment/business property. Or, always meant to be managed as Rental. Or...well, that's why more details will matter.

It was never meant as an investment property it was rented for a couple of years while owner was transferred to Switzerland in business. It was rented in 2020 but not at all in 2021 and sold in 2021

Sounds like it was a second home at the time of sale. Not a business property. All gain taxable. Gain to the extent of depreciation claimed is unrecaptured 1250 gain. No loss allowed.