A perpetual licence means that if you buy a perpetual licence for Vegas Pro - currently version 22 - you can use that version forever. You will be entitled to get any updates for version 22 but you will not be entitled to get any later releases of Vegas Pro (e.g. VP23, VP24, VP25 etc). To get a later Vegas Pro version, you would have to purchase it for which you should be eligible for upgrade pricing.

With the subscription option, you'd get any new releases of Vegas Pro while the subscription is active, but if the subscription is cancelled, then there will be no further access to Vegas Pro at all. The option in that case would be to either to restart the subscription or to buy a perpetual licence.