My personal understanding: the US textile industry insists yarn-forward RoO in TPP is not because they expect a substantial increase of textile exports to Vietnam as the case of NAFTA and CAFTA which help capture the export markets in Mexico and Central America. But rather it is because:
1) Without yarn-forward, situation will get even worse. Particularly, a less restrictive RoO will make Vietnam’s apparel exports which contain textiles made in China, Taiwan or South Korea qualified for duty free access to the US market. Definitely this will be a more imminent and bigger threat to the US textile industry than simply facing competition from Vietnam’s apparel which contains Japanese made textiles. And still many US textile companies don’t treat the Japanese textile industry very seriously, although I think they should. Remember, Japan currently is the fourth largest textile supplier to Vietnam and the NO.1 textile supplier to China.
2) With yarn-forward RoO in place, at least US textile companies can invest in Vietnam (remember, globalization is about movement of capital as well. Many apparel companies in Mexico and Central America actually are invested by US companies). Without yarn-forward RoO however, Vietnam can simply rely on imported textiles as the case mentioned in (1) and there will be no incentive for US textile companies to move factories to Vietnam (meaning, capital holders will lose).
So overall yarn-forward RoO may win a few more years for the US textile industry. But in the long run, it is my view that the US textile production and its exports to the Western Hemisphere countries may still inevitably decline (especially those output to be used for apparel assembly purposes) after the implementation of TPP. In the 21st century, the nature of competition is supply chain v.s. supply chain.
The future of the US textile industry is those high-end markets, particularly technical & industrial textiles.
Additional Reading: The potential impact of TPP on the US textile industry