If Japan signs the TPP, that won't prevent her from forming an FTA with China. For example, Australia -- she signed an FTA with China recently --- yet she's still signing the TPP.
Vietnamese buddy, while the TPP enables Vietnam to have greater access to the markets in say -- US, Australia, New Zealand, Japan et al -- it doesn't necessarily mean that Vietnam will abrogate its trade agreements with China, in fact, there is the potential of Vietnam and China signing a FTA in the foreseeable future.
Second, Tho the markets in TPP will be an economic boost to Vietnam's agricultural and industrial components -- these will only be a synergistic benefit to Vietnam. I don't see any change to the $30 Billion bilateral trade between China and Vietnam, in fact, from reports -- this will only increase in 2015 and beyond.
In discussions regarding national development , intraregional developments -- its important to divorce nationalistic sentiment.
Thank you and take care.