Dear Matt,
I don't think it is a dodge on VN's part.
VN may not have learnt the word "skoramis" from Browning; and what VN meant by "English dons in the past" may refer to his Cambridge days.
When it came to rare words, VN was the first to acknowledge their source, like in the case of "mollitude" which he used in his Eugene Onegin translation (and later on in ADA, Glory and Ultima Thule), and defended by stating that Browning had used that word. In fact, Browning used the adjective "mollitious" in Sordello and The Ring and the Book.
Kinbote believes Shade borrowed the word "stillicide" from the poem "Friends Beyond" by Thomas Hardy, but the same word we already encountered in Invitation to a Beheading. Besides, VN was a diligent reader of dictionaries.
Regards,
A. Bouazza.