So as is pretty obvious by his design, Lạc Long Quân is pretty heavily inspired by Sol Badguy, and that goes for his weapon too. That big hunk of metal fashioned as some sort of sword/axe is in fact merely a weaponized casing for a far greater weapon. And that's where I'm actually at kind of a stand-still in regards to what to do.
The idea I have for him is that since he's regarded as the "Father" of the Vietnamese people, he has "authority over all other Vietnamese Heroic Spirits/Servants, and thus is able to use their Noble Phantasms too (kinda like Gilgamesh's collection in the Gate of Babylon, just on a much more limited/localized scale). That leads me to a certain sword in Vietnamese Legend; Thuận Thiên. Funnily enough, that sword has made an appearance in a Type-Moon work. If anybody remembers the Fate/School Life manga, there was a contest to submit ideas/artwork for servants, and one of the winning entries was Lê Lợi (appearing as a Saberface dressed in an áo dài), who even shows up as Himuro's servant in the animated short for New Years five years ago. Now, the thing is, should Thuận Thiên simply be another divine weapon, or would it be acceptable to have it be Excalibur appearing with an alternative shape, considering the similarities of their legends? I have the idea that, considering the fact that Vietnamese people see themselves as descended from a dragon (Lạc Long Quân), Lê Lợi would have been someone whose latent dragon factor was surprisingly strong, and therefore be able to wield Thuận Thiên/Excalibur.
And working backwards from there, Lạc Long Quân's legend has him slaying several demons and monsters, including a giant fish demon (Ngư Tinh) and a nine-tailed fox (Hồ Tinh). And in these cases he cuts their heads off or more, which would probably need a pretty impressive sword.
Now moving on from all the sword talk, I can at least present another of Lạc Long Quân's Noble Phantasms:
By fully tapping into his draconic nature, he transforms into a golden dragon, before he annihilates his foes with his full might, either by using his Dragon's Breath to launch a torrent of pure magical energy, or just bumrushing the target like a Shine Spark.