By default vim opens .ss files with some other file format syntax highlighting.
To enable HTML (actually XHTML) syntax highlighting on your .SS Silverstripe template files, create (or edit) your ~/.vim/filetype.vim file. Then enter this:
au BufNewFile,BufRead *.ss setf xhtml
Then open a .ss file and it’ll give you nice HTML syntax highlighting. And because it’s in your home directory, it’ll keep working even after you upgrade vim.