Can I substitute vodka for rum in tiramisu?
Rum used in tiramisu is mainly for flavor. It's not a fundamental ingredient that will completely change the outcome of the recipe. 

That being said if you wish, you can omit the rum without adding the vodka. The vodka will add nothing to the tiramisu recipe since vodka is made to be flavorless. 

As Jefromi mentioned in the comment, there are plenty of other better substitution in terms of flavor you can use. Kahlua is a very common liquor found in tiramisu as it's coffee based. Chocolate porters might also add a nice touch.