Was curious if I can start new mandavillas by simply planting one of their runners in the soil?

Lori, Sure can as long as you include a leaf node. I would just leave the runner attached to the mother plant than lay a runner in soil along side it. Once roots form on node, cut new plant off. Or just could take cuttings. Use rooting hormone. keep at 70-75 degrees F and growth should begin in about a month. good luck.