The ornamental design for a vehicleseatbasecoverskirt, as shown and described.
FIG. 1 is a front perspective view of a vehicle seat base cover skirt showing my new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a left side elevation view thereof; and,
FIG. 8 is a right side elevation view thereof.
The broken line showing of a seat-top part and backrest part are included to illustrate environmental structures and form no part of the claimed design.