Is this slot on the Navon, or is it on the Landing Page item?
In either case, Slots that were implemented in the XSL assembly model should work just fine in Velocity. You refer to them with the same Macros that you do with newly created slots.
If the slot is on the Navon (so that it applies equally to all pages in the section), then you just need to add one of the #slot macros to the “NavLeft” template.