On my Type B vouchers with profile name (upper right) Dollar-UA-Systemwide there is on the back, under Voucher T&C 1) Valid Carrier: This voucher may be applied to United, United Express, or Ted ("United") operated flights, but not to flights operated by other airlines (such as United-marketed code share and Star Alliance flights) and 2) Valid Routing: To take advantage of this voucher you must be traveling on United for your entire intinerary and you must travel using a permitted routing

This was also confirmed to me by United RES. These rules look fairly clear for vouchers at least under this profile

Of course you might follow the advice of zrs70

Jeff