At
[VHDL] Syntax Highlighting example a wordfile for syntax highlighting is posted.
You can see there that only the double quote character is specified as
String Chars = in the first line. I guess, in your language definition for VHDL either
String Chars = is missing or " and ' are defined as string characters. Remove ' from this definition.
Additionally you should insert in the first line for example between
Nocase and
String Chars = the keyword
DisableMLS (with exactly this spelling!). This keyword disables multi-line string highlighting. Well, I'm not a VHDL programmer, so I don't know if VHDL supports multi-line strings or not.