Encourage A4 to page break before a tune. This enourages page breaks to happen before tune start, and not between the tune image and any following wording. Also move the vfill to the tune start. That way, if the break does not happen, the vfill will, and the following tune will be spaced down to the bottom of the page if it's the only other tune on the page. Otherwise everything tends to clump at the top of the page.

X:1
T:Sir Roger de Coverley
M:9/8
Q:180
K:G
"G" G2G G2e dBG | "D" A2A ABG FED | "G" G2G G2e def | g2G "C" GAG "D" FED :|
"G" dBd "C" ece "G" dBG | "D" E2A ABG FED | "G" dBd "C" ece "G" def |\
  g2G "C" GAG "D" FED :|
"G" gag "D" f2e "G" dBG | "D" E2A ABG FED | "G" gag "D" f2e def |\
  "G" g2G GAG "D" FED :|
"G" dcB "Am" cBA "Em" BAG | "D" E2A ABG FED | "G" dcB "Am" cBA "Em " Bcd |\
  "G" g2G "C" GAG "D" FED :|