Although you already mentioned it, the interesting thing about the pettex gravel mentioned is that is says the following:
Decorative aquarium gravel is the ideal way to enhance the appearance of you fish tank. Made from silica aggregates known for their inert properties and coated in a colourfast resin for trouble free fish keeping.
Suitable for all types of aquarium set-ups including coldwater, tropical and saltwater.
Roman decorative aquarium gravel is also the perfect planting medium due to its finer size.
Suitable for bottom feeders and barbed fish.
Increased surface area allows for improved anaerobic activity.
Will not change the PH of the water.
Available in 2kg and 8kg packs
So the fact that your aquarium gravel "reacts quite violently with vinegar" just doesn't make sense. Sounds like total false advertisement...unless the "roman'" gravel isn't the same as standard pettex gravel. I see it is recommended for plants...although it appears only due to it's relative size.
Did you contact the company about your issue? It would be interesting to see their reply.
BTW - I use eco complete and I would say that it is very much like sand.