A combined experimental and theoretical study of transition metal-free vinylations with the recently discovered hypervalent iodine(III) reagent VBX has revealed two different pathways leading either to the internal or the terminal alkene. Deuteriumlabelling studies and computations support that the S-vinylation of thiols proceeds through deprotonation followed by a ligand coupling to provide the experimentally observed internal alkenes with retained E-configuration. The P-vinylation of diarylphosphine oxides instead begins with I-O coordination of the corresponding phosphinous acid to VBX. A simultaneous deprotonation and Michael-type addition then gives an anionic intermediate, which is transformed to the terminal alkene through a base-assisted protonation and E2 elimination. A general regioselectivity trend for VBX vinylations under metal-free conditions is predicted, where ambident nucleophiles will deliver terminal alkenes whereas monodent or strong nucleophiles will provide internal alkenes.