99.9999% sure the issue comes from where DaveUK says. I’m no longer a Vesuvius owner but used to have one during six years or so, I used it a lot. Minimum 6 to 8 espressos everyday. This seems to be a common issue with this machine as other mates reported exactly the same. That one way valve seems to be very sensitive regardless the water you are using. I used to put soft water, no more than 6ºF and about 70ppm. A friend using osmotic water corrected with bicarbonate had the same issue. Both of us used to descale the machine every 12-18 months or so.
You can test this in a very easy way, no need to look for the feed tube. When you make the last espresso of the day, mark with a pen or a sticker the water level on the deposit. Next day, when machine has been hot an iddle, check the level again. If the level remains the same, the machine will work normally. If the level has increased, you will noticed the behaviour shown in your video.
Good news is that doesn’t seem to be dangerous for the heating element, probably it remains covered with water. I can’t be sure of this but my heating element didn’t suffer any damage and this happened in my machine for years. It was such a nuisance.
Bad news: that one way valve is veeeery difficult to take apart. It’s probably the only part of the machine which is glued. Apart from this: I descaled and clean the machine at a workshop suggested by acs (machine was clean and scale free anyway) and the whole brass piece, that masello di carico, was replaced with an acs spare. Despite this, the water still returned oddly from the boiler to the deposit.
Good luck!