Grace notes and doubled bar line

[David Sumbler]

\version "2.19.48"

\language "english"

vn =
\new Staff \relative g'' { 
  g1 \bar "||" \time 6/8 \tempo "Vivace"
  R2. | \grace { g16( a } g4.) g |
}

vc =
\new Staff \relative g' {
  g2 g \bar "||" \time 6/8 \tempo "Vivace"
  \grace { g16( a } g2.) ~ | g |
}

\score {
  \new StaffGroup <<
    \vn
    \vc
  >>
}

The above compiles to give two double bars at the time change, each
complete with new time signature and tempo marking.  The grace notes in
the lower part are printed between the 2 barlines.  I want the grace
notes to appear after the barline as usual.

The funny thing is that if I change the second line of the upper part
to "\grace { g16( a } g4.) g | R2. |", so that the upper and lower
parts both have grace notes immediately following the double bar, it
compiles as I would wish.

Where am I going wrong?

David