is it really safe to let one fuel tank dry out? Isn't fuel system on most aircraft designed with redundancy in mind shall there be a vapour lock
Yes, it is entirely safe to fly one tank to exhaustion, as long as you have useable fuel in the other, and you are prepared with the proper procedure to restart the engine if need be. Vapour lock is not at all associated with running a tank dry. If there is a restriction to fuel flow or vapour lock, an empty tank will have not been the cause. In situations where you intend to be aloft for a very long time, running one tank to empty can actually be reassuring to some degree, because it gives you a better sense of how much fuel you have at your halfway point, and thus how much time/distance is still possible.
The PA38 fuel system, like all low wing systems which come to my mind, does not have a "both" selection available, because a certification requirement for aircraft which are to have a "both" selection available, is that the airspaces of the wing tanks be interconnected with a vent line, and that is just not possible. A vent line would have to connect the outboard upper area of the tanks to each other, and then not have a dip in the middle, where fuel would sit, and prevent proper venting. Not physically possible.
A few high wing aircraft do not have a both selection (200 series Cessna come to mind) I don't know why Cessna designed them this way, but I'm sure they had their reasons. There certainly have been crashes because 100 series Cessna pilots flying 200 series Cessnas forgot to use the other half of the fuel they were carrying!
Cycling the fuel selector through the selections from time to time is a good idea. It keeps things from seizing up. If taking off with fuel selected to "off" is a concern, only allow pilots who preflight check fuel selector positions to fly!