As for vibration, you would not enjoy the leuku buzzing like an angry bee when you hit something really solid, and stinging the hand.

A heftier blade with decent grind will bite deeper and gradually halt, relatively speaking.....likewise, a thick blade with poor blunt edge would also not be a pleasure. But heavier bites deeper in general, everything else being equal.

Back to the rhomboid, I have read puukko theories that it developed from using old files, and that shape simply copied. I do not buy it because it would be extra work to no real purpose.

Personally, think it came about to save work. The centerline of blade not hammered out as much, and mainly kept blade fairly straight with minimum of forging. And advantages discovered such as centerline strength, mass maintained above edge, but less drag or wedging in cuts and chops, more manueverable in deeper cuts and more complex slicing/carving. The rhomboid on a leuku is slight, but it is there. However, most folk would not spot it unless measured, and most writers until of late have concentrated only on general shape, so do not expect histories to mention such subtleties.

No doubt, parallel side knives also made, likewise grinds varied between convex and flat, likewise secondary edging, depending on smith, depending on customer, depending on custom. Through near 1500yrs of making, though, many would have had definite ideas about what worked in that environment, and why. Leuku stayed fairly constant for hundreds of years until the industrial age and consumerism struck.