Fix an OOS on rejoin on Danubius following rP20957, refs #4855, fixes #5198.

Vectors are still deserialized as objects without their prototype functions, so doing math on these obejcts fails upon rejoin, refs #4698.
This patch doesn't add any version incompatibilities and players with the patched version can successfully rejoin unpatched games.

Differential Revision: https://code.wildfiregames.com/D1548
Reviewed By: temple