I think Vietnam still need to develope new tanks along with new tank guns (bigger caliber). Having RPG-29 is also good since its very convenient in close combat or in the city. But its going to be hard approaching a tank in open ground.
Additionally i just want to mention that PG-29 is a HEAT type ammunition based on munroe effect. Which will shoot out a high pressured stream of metal crumbs when its triggered, and drill through armour. Since PG-29 is two-staged, it would be good to aganist a tank has plain steel armour even with some ERA (like Type 59 equiped with some ERA), chance to get through is high, and very possibly to kill personal with the high speed metal crumbs and leaves a mess in there.
View attachment 153912
View attachment 153909
But it would have less chance to break through composite armour tile (not impossible, but less chance, depends the location and bit of luck), eg. for ceramic armor, the energy of the pressure will distriputed more evenly on the surface which leaves less follow-up force to contining penetrate. Not to mention tanks would equiped with cage armour, ERA, composite armour, even chains when they enter battle.
View attachment 153921
(Chain hanging on the back like Merkava MK4)
So i would say for now kinetic energy weapon like APFSDS, or ATGM is best way to go, and I think Vietnam has already got some from Isreal (For ATGM is possibly LAHAT, not clear about the amount).The the main difficulty for T55 changing its gun to 125mm is the balance issue, the space for autoloader, which decrease the ammunition-carrying-ability. And the overall modification cannot exceed the weight limitation, good mobility still needed, cuz T55 doens't have enough room for a new engine, but 105mm gun is the best fit for now.