I am studying modal shifting, borrowed chords and secondary dominants, and I am a little confused of their function, if they do not resolve their own tonic. Would their function then just be a borrowed chord? For example I have the following progression. I V7/vi IV IV I V7/vi V IV I V7/vi VI VI I V VI VI
Since the V7/vi, is not followed by the vi, what is its function. Is it still a secondary dominant? Is it classified as a modal shift, or a borrowed chord? This chorus section is in D. The verse is in Bm. Which I realize is the vi chord of D, which might change the whole analysis, itself. I’m a little lost. Any help would be much appreciated. Thanks!