Well, in addition to the cutters, the US would have quite a few WW2 DEs (which OTL S Vietnam got a few of) & Auk/Raven class minesweepers available for free or cheap that could make for the larger combatants & patrol craft, & in fact were exported to a number of US allies in this timeframe for just that purpose (and some even still actively serve in those roles in several navies to this day.) If the budget & manpower situations would allow it, they might even be able to swing a couple Benson or Fletcher-class destroyers for pretty cheap as well. They'd probably be good enough to serve most needs as S Vietnam's major surface units other than fighting against modern submarines or facing down air attacks from jet aircraft for a decade or two until a sufficient number of modern frigates can be acquired, assuming your South Vietnam fares rather better than the OTL one.
If there's going to be a lot of coastal/riverine operations, don't forget the amphibious forces- again, the US would have a decent selection of WW2 surplus from small landing craft all the way up to LSTs available for cheap, & some of the landing craft can be converted into river gunboats & troop transports as well like the US did OTL.
Not familiar with what sort of small patrol craft are available at this time to supplement your armed diesel junks, other than the Norwegian Nasty-class PT boats, which were pretty nice as torpedo boats or fitted as fast gunboats, as the US has pretty much disposed of its WW2 leftovers in this area go, so you might want to poke around Wikipedia for a bit & see what sort of craft in this class are being built by European navies at this time for their own use or export.
Don't think anyone;s building dedicated riverine warfare craft at this time, so if something other than converted landing craft & civilian vessels is needed, your South Vietnam and/or their foreign advisers would probably have to get someone to design & build craft such as the OTL Swift boats & PBRs, & anything else that might seem useful.
And don't forget the supporting stuff, such as a couple tenders/repair ships to support the operational units, a few tugs, some coastal minesweepers, a salvage vessel or two, & so on- most of that stuff would be available from WW2 US surplus for cheap, or commercially available civilian vessels bought into service & appropriately refitted for all but the minesweepers.
If transports, cargo ships, & oilers beyond the amphibious forces & converted diesel junks are needed, I'd imagine that a good number of those would be coastal freighters & tankers bought into the service, as although there's a good deal available from the US, a lot of it, such as say, a T2 fitted as an oiler, or a C3 fitted as a stores ship or troop transport would likely be overkill for most needs of a force with a coastal/riverine focus.
Given that focus, a small force of marines that's part of the naval service at the political level but otherwise its own service, designed around boarding, coastal & riverine landings, & a spec-ops component would be useful, say a brigade or 2 to start, as there'd likely be interservice cooperation problems with designating an army unit for that, & a naval infantry force could become kind of the ugly duckling of the navy.
But really, if surplus WW2 hulls would suit most South Vietnamese requirements for a while where it comes to building a navy, then hooking up with the US military assistance programs for this case can make a lot of sense where it comes to getting a bargain on at least some of the hardware.
