Unfortunately, I haven't been to T4 yet, but I think this is hardly doable at all. Consider that your DRW flight is on time for gate arrival. You may need up to 10 minutes to get from the gate to immigration. With another 10 minutes of waiting in the immigration queue, that leaves you 20 minutes to a) get landside, find the shuttle to T4 at an unfamiliar airport, b) physically get to T4, c) get to VN check-in counter.
You have slightly better chances if you check in online for VN, but personally, I would only consider a 90-minute cross-terminal transfer only at an airport I am very familiar with, and have full knowledge of how to get there. On separate tickets, I would not consider it at all. With your timing, you only need a 20-minute delay (which can easily be spent holding above Changi because of high traffic) to thwart your plans.