You can add custom syntax definitions to Textastic by using TextMate bundles. Seehttp://www.textasticapp.com/v6/manual/lessons/How_can_I_add_my_own_syntax_definitions__themes_and_templates.html
A quick search for "Vala tmbundle" in Google brought up this TextMate bundle which should work:
https://github.com/technosophos/Vala-TMBundle
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You can add custom syntax definitions to Textastic by using TextMate bundles. See
http://www.textasticapp.com/v6/manual/lessons/How_can_I_add_my_own_syntax_definitions__themes_and_templates.html
A quick search for "Vala tmbundle" in Google brought up this TextMate bundle which should work:
https://github.com/technosophos/Vala-TMBundle