I wasn't around for the Vietnam War, so my take comes from other people's accounts and opinions.
The fall of Kabul was broadly predictable. Either as it happened or via the "enemy" melting into the existing leadership. Corruption is king. In that sense, Saigon's fall feels similar: corruption ran wild in both governments.
Who suffered most? The people did, in both places, so if this is the intent of the question it would be roughly equal unless you want to compare body count.
The cultural contrast is stark. Without getting too "non-PC," I think the Vietnamese, then and now, had more going for them than Afghans do today. For the U.S. population, Afghanistan is widely viewed as a largely insignificant country with near-zero expectations for a prosperous future. Daily life for many Afghans looks much the same before, during, and after the recent wars—just with the scars of war piled on top.
I don't know exactly how Americans viewed the Vietnamese during the war, but post-war Vietnam has proven capable. Given its neighbors and trade position, it's in a good spot. Vietnam has prospered; I don't expect Afghanistan to follow that path anytime soon.
I've been to Ho Chi Minh City (Saigon), and it's strikingly modern. Think San Francisco, but with better weather, much cleaner parks, sidewalks, and streets. I felt completely safe. I have not been to Kabul, but I fully expect it to not be comparable to Saigon of either the early 70s or today.
That said, flying in at dusk, I saw a patchwork of fires across the countryside. Who knows what was burning. And it didn't inspire confidence about the air.