I can see vcf support in several galaxy files (datatypes/tabular.py, datatypes/converters/vcf_to_summary_tree_converter.py, visualization/tracks/data_providers.py...) but it is not visible in the main web interface (i.e. I can't assign VCF as type and it is not automatically recognized).... what can I check to make this properly work?

You need to update your datatypes_conf.xml file to include support for VCF: