Oxford defines virtue as "behavior showing high moral standards." I think assertiveness only conditionally falls into this category.

First, as noted above, assertion to one can be aggression to another, thus questionable high moral standards. However, if both parties agree it's not aggressive, then is it indeed morally good to be assertive? To me, morally good is a higher bar than simply being non-aggressive.

I think if virtue is defined on moral grounds, assertiveness is not consistently enough placed into that category.