What would I gain by upgrading a norco vfr 2 hybrid bike group set from alivio 9*3 speed to a tiagra or 105 shimano group set? Are there any things to consider or look out for?
Things to Gain:
Higher quality groupset
It is not really a good idea for this type of bike to consider trying to evolve itself into another type with the components your suggesting
The Norco VFR 2 Hybrid has the Norco 25mm Riser and handlebars which are not compatible with Tiagra or 105 shifters.
Tiagra and 105 are both STI (Shimano Total Integration), This means that they are not designed to fit this bike handlebar style and are designed for drop style handlebars
Trying to keep your original shifters will probably not work as the indexing will not mesh. Although there have been some reports of users getting lucky mismatching shifter and derailleur models, it is generally not a good idea and leads to frustration and ending up with brand new components you spent good money on that just don't work.
Brakes are another story. Trying to match your existing levers with tiagra or 105 calipers will most likely lead to more frustration as chances are, you will never get them adjusted perfectly to your liking and the original brakes may feel more natural and function better.
105 does not come in 9 speed so you will be limited to Tiagra.
You can not upgrade to a 10 speed group without changing out your rear hub which would mean a new wheel or wheel-set.
You mentioned 9/3 drivetrain. I imagine that means a triple front crank which is not compatible with tiagra or 105 and would require a crank replacement.