Just a follow up. We didn't have any problems in Ho Chi Minh regarding the TWOV. We told the gate agent our intention and she didn't have any issues even though we flew to Vietnam on a separate ticket. Once we arrived in Shanghai, we didn't have any issues either just a very long line with strict agents (not surprised). We also had a separate ticket flying from PVG to SEA. Personally, I would rather go to Hong Kong or Tokyo next time rather than deal with this policy. Not a huge fan of China.