judging u only have a week there you can't really go from city to city. those angmoh backpack to travel from north to south or vice versa take like 1 month. since it's your first time to vietnam, u can either choose ho chi minh or hanoi. ho chi minh if you prefer the city, hanoi if you prefer nature. both have nearby cities to travel out to if u have time. hcm can go to vung tau or dalat, hanoi can go to sapa.
i really don't suggest Ha Giang loop on motorbike because firstly, i think it may be a bit rush for 1 week. secondly, it's a bit too dangerous for first timers to vietnam.
if you don't have a motorbike license, i really suggest u DO NOT ride a motorbike, especially for a first timer to vietnam. traffic conditions are very different from singapore, and u need to think about insurance coverage as well. besides, if you google about it, u will find information about police everywhere being more strict on foreigners this year, so it's also something to consider about.
Ha Giang is full of winding roads so it can be dangerous for first timers. you can consider this kind of adventurous trip if you have more people with you that are already experienced in vietnam or locals that can communicate easily in case of any trouble.
just be careful if you're still intending to do it.