As above, though you could mix(no pun) between mono and stereo. IE; the Kit can be routed to a pair in stereo and pan the group hard left and right,and then the kit chans can be panned to suit as normal, this can help create a bit of space in a mix.
Then you can route bass and gits to one mono sub group and the vocals to the other.
One thing to consider when grouping is dynamic treatment, like compression.
I often favour a stereo sub group for vocals with a stereo compressor in the group inserts. This can give your vocals an even tightness and help sit BVs in a mix too.
This works in mono just as well.
Of course, your desk needs to have insert points on the groups for this.
Another trick you can use is route the main vox to a group AND the stereo bus as well, it`ll add a db or two
If you have a spare chan, use a mic splitter cable and cable the main vox to two chans, use one for FOH and one for the Aux monitor send,the EQ on the 2nd chan can be used to get the best tone for the monitor wedge, leaving the FOH chan EQ for FOH, it also means if there`s any compression on the FOH chan it won`t be heard in the wedge (as some prefer).