Can I heat the rice wine vinegar with the sugar,as you would do in sushi rice ?

You could heat it, but it would affect the texture of the cucumbers. The warmer the vinegar/sugar mixture is when added to the cucumbers, the faster they will absorb it. That will make them lose their bright green color, and they will become softer much faster. You could heat it slightly, just so dissolving the sugar is easier, and let it cool back down before adding to the veggies. That would help preserve the crispness and color.