Stupid me!! I also forgot about this option...... I use it a lot for British Airways flight when I want to get an overnight connection in London but I forgot about this option when looking on Vietnam Airlines website....... Aaaaah, if only I haven't booked them on Monday evening..... I'm so mad now at myself. Especially as the price for multicity is exactly same as my price for two separate tickets - it would so much easier to fill in the forms only once (and as I was buying ticket for a party o 4 people it was a lot of typing in....).....
Thanks dici for realizing my stupidness. :-(

By the way - I tried to contact VN online support via e-mail and on Facebook - they are not able to give the answer about the baggage through checkin. They say "our team will do their best to assist you, but if they can't than you will need to collect your baggage at SGN" - so even they don't know their policies... :-)

Thanks Firemansam and dici for your contribution in this topic and helping me with my query. I wish I hadn't booked them before I had an opportunity to read dici's post. I read an email notification of your post and contacted the VN if they can cancel my ticket for full refund as it's less than 24hrs after booking (as all IATA airlines need to do for booking from/to US) but they refused to do so.
So I'm finally with two separate tickets and 2:40h connection in SGN - hopefully there won't be any delays on the first flight and we can proceed with our journey smoothly.