At -78 degrees C and in apolar solvents, the presence and quality of a coordinating side arm in vinylsilanes dictates both their reactivity toward alkyllithiums and the structural motif in the solid state, deciding whether a defined carbolithiation, slow polymerization or no reaction at all occurs. The crystal structures of the carbolithiation products of two vinylsitanes and methyllithium are presented, and the energetic barriers of the addition step with different alkyllithiums are investigated.