A P-1 or P-8 is overkill in this role, even an old P-3C or a C-295MPA is more than sufficient. There are many other smaller MPA that could be more suitable in this role, though VN requires significant range so a DA-42 might be too small (the coastline is 3,000+ km long!).
Yes, I would think that VN will choose the P-3C or the C-295MPA for non-military purposes. The jets (P-1/8) are too expensive and unnecessary, while the little DA-42 might be not enough, as it cannot fly PQC-CAH-VCS-PHH-CXR-UIH-DAD-HUI-HPH nonstop (the estimated VN-coast route).
Similarly, additional transport capabilities can be used for disaster relief, medical transports and domestic troop or police movement. Some new helicopters would be a good investment for both land-based units and the coast guard. I've found many places to be well connected by road but off-road terrain can be difficult to traverse (often mountains, jungle, or both) and tropical weather can easily wash away some roads. Earthquakes are not a risk in VN.
Yes VN should expand its C-295 fleet as well as the helicopter-fleet.
The VN non-weaponized fleet of helos is now pretty acceptable for transporting purposes. The problem is its very-small fleet of just only 6 of C-295M, which are obviously not enough after the retirement of 20 An-26.
Combat aircraft such as the Su-34, I struggle to justify. VN's other neighbors, Cambodia and Laos, have no significant air force if any at all. The multirole fighters that VN already has could be focused on ground-attack if they wanted to operate there. Both states are so poor and ill-equipped, though, that they wouldn't dare attack Vietnam in its current strength.
Across the sea, Thailand and the Phillippines have some military strength but I have not heard of any political conflict that could trigger a war. Again, I'm not too familiar with the political situation in this area, but the most likely controversy would likely revolve around control of the ocean, for which coast guard ships and patrol aircraft are more suitable.
Air-2-ground attackers are having an important role in VN strategy. The Su-22 attackers fleet helped VN controlled half of the Spratly Islands in the 1980s, and the attackers would also act as supporters for the navy vessels also. Of course, the multi-role Su-30MK2V can be good enough, but obviously, VN wants something more due to its demand to re-invading islands and attacking low-objects.
VN is having 34x Su-34 alongside 35 multi-role Su-30MK2V. The aging fleet of Su-22 can be ideally replaced by a new fleet of 8-12x Su-34 attackers and 12-24x multirole Su-30MK2V/SM or Su-35S. I am expecting that VN would buy 12 Su-34 and 24 Su-35S, and it would be a shift from an attacker-heavy fleet to a fighter-relied fleet in any way.
(the pure-attacker fleet dropped from 34 Su-22 to potential 12 Su-34;
meanwhile, the dogfighters/multirole fleet grow from 11 Su-27S + 35 Su-30MK2V = 46 fighters
to 46 + 24 = 70 fighters after the suggested purchasing).
And yes, if there is any kind of formal conflict that Vietnam may engage in, it is only the South China Sea. China could also manipulate Cambodia to give some pressure on the Southwest border (near SGN) and in Phú Quốc Island (PQC) but of course not so significant as a comparison to the "East Sea" (VN's local term for the South-China Sea)