I see the problem.
You need to have a _0 and _1 in the NAME property for a two part map. Do as below and you should be golden.
<ImageStyle Name="exp02_dun_shrines_of_sky_epic01_vaults_0" zonerect="-107, -45, 139, 171" heightmax="20">
<ImageFrame Name="imageFrame" Source="images/maps/map_exp02_dun_shrines_of_sky_epic01_vaults_0.dds" SourceRect="0,0,436,506"/>
</ImageStyle>
<ImageStyle Name="exp02_dun_shrines_of_sky_epic01_vaults_1" zonerect="-107, -45, 139, 171" heightmin="20">
<ImageFrame Name="imageFrame" Source="images/maps/map_exp02_dun_shrines_of_sky_epic01_vaults_1.dds" SourceRect="0,0,436,506"/>
</ImageStyle>