make a model with Single Color RED as the string type (not RGB)
Make it single line with 1 string of 1 node.
Put colorwash on effect 1 with RED, put colorwash with BLACK on effect 2
select Effect 1 for the layering.
click morph.
1 channels will be used. It will Start at 255 and fade to zero eactly however long there is between each row.
You will see it start with full on RED and then fade t0 BLACK by the next beat. It will then go back to Full RED and repeat on every beat.
Export as a vir file and paste into your vixen sequence.