what are the importants of transformer vector groups?

Answer 1

None, if the transformer is only supplying a load which cannot be interconnected with a supply from another transformer.
The most common vector group is DYn11 which signifies that the primary (higher voltage is connected delta, Yn statest that the secondary voltage is connected in Star (Wye) and has a neutral brought out to a TXFMR bushing and 11 shows that the its vector is at 11 o'clock or 30degrees before the 12 o'clock position.
When you have two interconnected supplies they must be of the same vector group, otherwise they will be out of phase and cause an overcurrent and trip.