Even though it’s a bass, the Fender Bass VI has always used vintage type guitar machine heads. (No, it’s not a baritone.) These are the typical Kluson-style, 6 In Line tuners with a split-shaft string post. And the reason Fender use this type of post for these basses is simple: the string gauge.
The current Squier Classic Vibe models ship with Fender’s nickel plated, steel strings that have a gauge of .024 – .084. This gauge is light for a bass machine head (typically around .050 – .105) but extremely heavy for a guitar tuner. To take the heavier string gauge, tuners with split-shafts have to be used. These posts can take up to a .086 string (around 2.2 mm) whereas a standard post (with a hole at the top) mostly takes up to a .060 string.
This does limit your upgrade/replacement choices if you want to keep to the original look and feel of the Fender Bass VI. Any of the following will work; Kluson Deluxe, GOTOH SD91, Wilkinson Deluxe. Make sure to check that the press-in bushings supplied have a large enough diameter to fit your bass.
To use a heavier string gauge or fit a modern tuner (locking mechanism, higher gear ratio, contemporary style), be prepared to do some modifications. You could purchase a set of modern guitar tuners and open up the post holes to the required diameter. Any of Hipshot’s Classic and Locking tuners for example can be comfortably widened to 0.100 (find out more HERE). You will also need to consider the headstock holes as they will likely be the tighter 8mm diameter (6mm post with an 8mm press-in bushing). A tapered reamer will be required to widen the headstock holes slightly to fit a 10mm sized machine head.