If you previously had droptabs, with vB version 4.2.1 to get them to work again, go to the ACP / Navigation Manager and edit the tabs that were dropdowns. In the settings you will see a field called, Target Menu, choose the menu for the tab like you did when you set it up with vBNavTabs and it should work again.
Or if you did not have hem before, to make droptabs in vB v4.2.1
It is a small process but not to difficult.
1) Create a tab.
2) Create a menu for the tab.
3) Edit the tab and find the setting, "Target Menu" select the menu you just created, click on save.
4) Add links to the menu you created, these are what will show up as the dropdown items in the tab.