Yes it can as long as you don't get the i3 model, that has 4GB soldered to the motherboard instead of 8GB. There would be no difference between an upgraded Vivobook and an Ideapad 5 that comes with 16GB.
The Asus Vivobook 15 OLED k513EQ though that you were looking at (K513EQ) does seem to come with 16GB already.
So to round up on the laptops we have:
- Asus Vivobook K513EQ
- Lenovo IdeaPad 5 15ITL05-A (I'm assuming this is the one with the MX450 GPU)
- HP Victus 15
- Lenovo Ideapad Gaming 3
- HP 15-ef2081ms
My two cents would be:
IdeaPad 5 15ITL05-A - Best all rounder - The best battery life of the bunch, relatively light for a 15.6" notebook and the MX450 gives it the option to do some light casual gaming on the side.
Asus Vivobook K513EQ - By far the most impressive display of all of them, some concerns about eye fatigue from the OLED panel with all day use. Battery life is a little on the short side but it otherwise has a nice balance of features and can do some light gaming like the Ideapad.
HP Victus 15 & Lenovo Ideapad Gaming 3
These two are in a different performance class to the other laptops, they are much better at playing games at the expensive of weight and battery life. I don't think it matters which one you pick from these two.
In terms of which one you go for it really comes down to priorities. If portability is a priority and it's going to be carried regularly in a backpack, it's going to be used regularly on the battery and it will be used extensively as a work machine. Then then the Ideapad 5 would be my pick.
If performance is the priority then the Ideapad Gaming or the Victus are more appropriate, if I had to pick I would go with the HP Victus on the basis it's more recent and doesn't have a hard drive. The Vivobook is tempting with the OLED panel because movies will look fantastic but I think the Ideapad is a more appropriate choice if the laptop is going to be used for long hours on a regular basis.
I wouldn't bother with the HP 15-ef2081ms, the screen resolution is too low and it's nearest competitor the Ideapad 5 has a better balance of features.