Re: Not so vulnerable...
[... if it took 20 bombs and 17 torpedoes to sink it. Just check what was needed to sink HMS Hood in a far shorter time.]
You really are comparing apples with pears in your statement. A few points to consider:
1. Hood was built nearly 30 years before the Musashi. Technology had moved on a lot in that time - no WW1-vintage battleship could ever realistically win in a straight-up fight with a WW2 (aka "modern") battleship.
2. Hood was a battlecruiser, not a battleship. Battlecruisers had heavy guns but thin armour, and were intended to fight and destroy cruisers, not battleships - the Battle of the Falklands in WW1 is a perfect example of how they should have been used. In the Battle of the Denmark Strait, Hood's real job was to smash the Prinz Eugene to scrap, not to take on the Bismark.
3. Hood had known flaws in her armour protection. She was scheduled for a rebuild to correct those problems in 1940 or so. Not surprisingly the rebuild was cancalled when WW2 kicked off.
4. Hood was at least sunk in a ship-to-ship fight, which is what she was designed to do. Musashi never fired her guns in anger; she was taken apart by a concentrated aircraft strike.
It is interesting to speculate what would have happened if the Musashi and Yamato ever came face-to-face with an equivalent US battleship force. I suspect that the USN might have found itself gravely overmatched - the 18" monster guns mounted on the Musashi/Yamato would have seriously out-ranged the American 16" guns, and even a single hit by one them would have probably caused major damage. Fortunately for the Americans, Pearl Harbour forced them to use a much more dangerous weapon - the aircraft carrier.