How to form an acetal from an enol ether?
Steps (Source: from comment made by Zhe)


Tosic acid protonates alkene to form oxocarbenium through resonance.
One of the alcohols from ethylene glycol attacks the activated carbonyl and toluenesulfonate deprotonates the oxygen.
Methoxy group is protonated by the regenerated torsic acid.
Methanol is eliminated via SN2 with the other hydroxyl group of the ethylene glycol.
Tosic acid is regenerated by deprotonating the 1,3-dioxolane group.