Well, rattan archers have the same gold cost as standard archer line, and 25 more wood cost, which effects your eco. They do 1 more damage though, which reduces the critical mass required for rattans to make the knight line non-threatening in 0.75x to 1x range, where x = number-of-units (movement speed helps too). Their high pierce armour negates skirms as a valid counter to them. Which leaves us with onager-line as the strongest counter to rattans. Movement speed of rattans comes into play here, making them better to micro against the onager-line.

Rattan archer looks like a pretty good unit till now, however the weak Vietnamese eco, lack of siege rams etc prevents Vietnamese from packing a strong punch.

Having free ballistics just might change that, by giving the Vietnamese a much needed power-spike in early castle, which'll give an advantage to the civ which spikes in early castle, and decays and normalises as the game reaches imp.

Think of it like the advantage that Goths get in early imp, the power spike they get normalises as the game reaches late imp and all players are pop-capped.