I'd avoid the vaseline.
It'll make the washer so slippy, that it'll stretch and slide out, before there's enough pressure to make a good seal (done that).
Washer, which is on outside of keg, doesn't have anything turning against it, so doesn't need lubrication anyway.
Clean off all the vaseline, with soap & hot water / meths / IPA.
Then tighten nut to (strong) finger tight. I (weak hands) usually tighten nut, with tap pointing to 4 o-clock, then holding nut still, turn tap to 6 o-clock.
With a new barrel, or when changing any seals, I now pressure test for leaks. Fill to neck with water, and inject some CO2 (I use S30 cylinders). Much nicer loosing water, than beer.
On my last barrel tap upgrade, to deluxe sparkler tap, I discovered the tap leaked (a slow drip) due to rough plastic moulding below the tap valve washer.
Replacement didn't arrive for a few days, so I had to reinstate the old tap for a last batch. It's washer, 35 year old, had quite a few splits.
PTFE between nut and spigot thread is a waste, sealing there alone wouldn't stop liquid going out under nut. Just leaves somewhere for any bugs to lurk.
Seal really needs to be the outside washer. The rubber should deform enough to seal against a fairly rough flange, but if too rough, could try a smear of (sanitary) silicone rubber under washer & barely tighten. Then tighten properly when set (after 24 hours).
Whenever lubricating rubber, or plastic, Silicone grease (lubricant) should be used (small tub available from screwfix, now also
available as a spray). Not vaseline, which rots rubber in long term.
It's worth putting some silicone grease on the cap/neck threads and o-ring, to make it easier to turn.