Charging with 2.4A charger ok?

I do so, without problem.

But I am not sure that it's the right way.

Sure, no problem. The basics in the all power sources. Amperes are the MAXIMUM current the power bank or charger can deliver. Each device connected draws the amount they need or can use. No more. Voltage is another question...

it charges much faster, that's for sure.
whether there will be long term damage, i have no idea.
there is also a thread here where someone left his va3 charging overnight plugged into a wall charger... next day it died...

If high amp chargers were detrimental then Garmin would have included a charger and said to only use that charger. But you can observe that every appliance in your house is fine as they are plugged into 15 or 20 amp circuits - but most appliances are rated at less then 5 amps - they all work fine.

Devices of any kind draw the power they need. The only danger is if they want to draw more then what the charger/circuit can provide.

Many USB wall chargers deliver the same amperage as a USB port in a PC - so calling out a "wall charger" doesn't really define much about it. If Garmin were worried about this at all, they would have not provided a USB charging cable that fits into everything from charges with 100ma output to some with 3A output.

well, a usb port puts out 0.5a.
a full charge takes me 1h15mins.

by a USB wall charger, means i used the one provided by my phones, so either a 1.8a or 2.4a...
and to go 50% took me something like 10mins.

Erm, I sort of agree and disagree here. My watch charges way faster if it is connected to my Mac USB3 port and recognized by Garmin Express (it when I see that segmented charge indicator), and it is mush slower if I connect it to another laptop that still provides USB power when it is off (just top of the watch screen says it is charging). What I mean that 1h15minutes charge is done with higher than 0.5A current.

its well documented that usb3 puts out 0.9a and usb2 0.5a.
the issue is not knowing the battery capacity of the va3... so its more of just experimenting to see what the time taken is to reach a full charge...

assuming the battery is 300mah... and 0.5a charging...and with 20% loss efficiency... that would take about 45mins to full charge...

so i can probably say while my usb2 port is putting out 0.5a ...it is highly inefficient... and definitely not more than 0.5a.