I don't know where you are from and I don't know about vetch but if it's green still it should work provided you give it some time and it has some growing temps, it will take longer than normal I would think.
I am still spraying roundup as of yesterday, might do a wee bit tomorrow before parking the sprayer. I think it will work well but I am only trying to kill weeds, not harvest them.
All the rules are out the window this year it seems...
AN ERROR DOESN'T BECOME A MISTAKE UNTIL YOU REFUSE TO CORRECT IT
Orlando A. Battista