What would be a better investment for portfolio diversification, HK Tracker Fund (2800) or Vanguard Emerging Markets ETF (VWO)?

It depends on your goals really. If we're looking purely to diversify, then VWO is definitely better since it has exposure to China, Taiwan, India, Brazil, South Africa, Saudi Arabia, Russia, Thailand and Malaysia among others, whereas 2800 is purely a HK ETF (intended to replicate the HS Index). However, if you're particularly bullish on HK/China equity, then well, 2800 might be a better choice for you.