I think the advantage the Harrier provides to the USMC is to do with interservice politics. The USAF and USN don't fly it, so there's no shared logistics chain, which makes the absorption of Marine Air into either of those service arms problematic. Additionally, Harriers fly from LHA/LHD assault ships, not catapult CVs, so their adoption guarantees that some flattops remain doing Marine things, not mostly doing Navy things and occasionally helping out the Marines when an avuncular admiral says so.
There was Marine air support in SEA - as demonstrated by the Navy stopping carrier operations at Dixie Station in 1965 because they were no longer needed as Chu Lai became operational. Harriers would allow a more flexble and faster response in CAS provision, yes, assuming additional airstrips were used in the South to balance their shorter loiter time - but the OTL approach was to have CAS aircraft loitering with tanker support all day anyway.
I guess the issues are - if the USMC fields AV-8s in Vietnam (and the AV-8A is not much aircraft to write home about), what doesn't get fielded instead?
Is it just trading one AV-8 squadron for one A-4 squadron at Chu Lai? Does that trade-off, which presumably allows a higher sortie rate per aircraft deployed, at the cost of a smaller bomb load per sortie, make for a better CAS outcome? Where does the time-per-sortie come from - if it's mostly in the maintenance/rearm/refuel on the ground, as opposed to the in-air transit time, do Harriers enjoy any advantage over Skyhawks there?
Additionally, the Harriers will pretty much be purely tasked with CAS missions, whereas the A-4s did go up North - how much will that lack affect the US's air war over Hanoi?