Are (high) pressure vessels in use now?
Oh yes - I may be wrong, but I'd guess that virtually every FCV on the road uses pressure vessels in the 5-10,000 psi range. If they didn't they wouldn't be carrying sufficient hydrogen to achieve a usable driving range.
The thing about hydrogen is that while it is flammable, it also is the lightest molecule (it is element number 1 in the periodic table) and the H2 molecule (i.e. two hydrogen atoms combined which is how hydrogen exists in nature) is very small - so it will pass/leak through just about anything except glass or fairly heavy walled metal or some specific types of polymer. That is actually one of the toughest problems that fuel cell engineers have been working on - preventing leaks so that the vehicle can actually drive a good distance.
To be more specific, something like a thin-walled aluminium tank would likely not contain hydrogen for very long at a significant pressure and if there was a leak, the gas would simply dissipate in the atmosphere and nearly instantaneously combine with oxygen to form water molecules. If a vehicle stored in a parking garage had a hydrogen leak, the gas would be gone before enough could accumulate to cause a significant fire. the escaping gas would simply pass right through the roof or walls of the building and be gone.
That is why the whole "Hindenburg" nightmare is simply not a realistic scenario. In contrast, gasoline, diesel or any other liquid fuel or a heavier gas such as propane or natural gas - can accumulate and result in a BIG fire - and that is why hydrogen (once they get the challenges around preventing leaks solved) is actually much safer than the fuels used today. Remember - no matter what sort of powertrain a vehicle has - it must still carry enough on-board energy to propel the vehicle down the road, up the hill and around the corners. There is no free lunch.
Finally, the amount of hydrogen on-board a typical car is likely to be about 5-6 kg versus 30-60 kg of gasoline carried by most cars these days. A big gasoline or LNG spill is a BIG deal and will lead to a BIG fire (see attached video).