I was reading about Jablonski diagrams and how they are structured. Bold lines would indicate ground electronic levels, and thin lines indicate the associated vibrational levels. Absorption only happens between the different electron energy (orbital) levels, however any additional associated vibrational level can be occupied (for example, transition from S(0) ground state to the third vibrational state of S(1)).
Why is this the case though? Couldn't a transition happen between the vibrational levels of an energy level (for example, from ground state S(0) to third vibrational state of S(0)) or does an absoprtion always change the electronic level?