Hello, I think the one I drew and included in the attachment is valid for Michael-type addition reactions. Do you make a favor to check the mechanism? I think, the steric hindrance of the more-stable carbocation in vinyl group can not participate in this type of mechanism, it is further suppressed by the persuaded electron flow coming from the neighboring terminal methylene, with the effect of attacking highly electronegative species. An intramolecular charge rearrangement gives a double methylene moiety from vinylmethylene. Also noted is the possible side reactions; can there be an ester exchange here instead of Michael addition? I think additions are more favored than substitutions, as it seems to require more energy. If I am wrong, I would like to know what you're thinking about this. Thank you for your interest.