Is there a free airport shuttle bus from HK airport to the ferry terminal/pier?

Yes, there is free shuttle bus service between HK airport and Skypier, where you can catch the ferry to Macau.

If you transfer at the airport to take a ferry forward, you don't have to go through immigration procedures or reclaim baggage. Just walk to the Arrival Level, T1 to buy ferry ticket and present luggage tag receipt at the Ferrt Ticketing Counter at Transit Area E2. After that, take the free shuttle bus to the terminal to scan ferry ticket and check your luggage status at the ferry boarding entrance. Then head for Macau.

whats the best way to get to ho chi minh city from hongkong

For Sigi: the best way is to take a direct plane from HKIA and it takes 2.5h or a little longer to ho chi minh city.