The ornamental design for a bottombase of vacuumflask, as shown and described.
FIG. 1 is a front elevation view of a bottom base of vacuum flask showing my new design;
FIG. 2 is a rear elevation view thereof;
FIG. 3 is a left side elevation view thereof;
FIG. 4 is a right side elevation view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a first perspective view thereof; and,
FIG. 8 is a second perspective view thereof.
In the drawings, the evenly dashed broken lines showing the remainder of the vacuum flash illustrate features which form no part of the claimed design.