VBS validation counting valid grades (redwood)

Dears experts,

we want Assignment Grade to be defaulted from Position Valid Grade.

This can happen only if a single Valid Grade is selection in Position, if multiple valid grades are selected no defaulting is reflected into the incumbent's assignment.

For this reason we want a VBS validation throwing a blocking message if count of valid grades selected into the position is higher than 1.

Is it possible?

Thanks in advance,

Domenico