Anyone?

there shouldnt be any vanes in the return duct. if there is, you should remove them. sounds to me like your return ducting is probably too small as well. vanes in a low pressure return duct system is not needed and noisy. I would remove them and make sure the duct is big enough.

oh and btw, from the looks of the data you provided your tech made some of those numbers up.
48 degree suction @ 80psi = zero superheat.
that 48 degrees is saturated temp.

The returns have grills over them, apx 8x12". Standard return grills like I see at home depot. Its the fins on them that vibrate.

He put 48 degrees in the column on the form for Suction Temp, but there is no column for Saturated Temp.

But back to the original question, is the slower blower speed reducing my systems ability to cool to its full potential?

The "delta T" is referred to as the split temperature - the difference in temp between the air before the a/c coil and after the a/c coil. Your split temps are good.

The slower air speed will provide better dehumidification and cooler temps coming from the a/c coil.

You can get the manual for the air handler from Lennox (or look at the wiring diagram inside the air handler) to find out what speed your fan is set on (low, med, med-high, high).

I believe that the "24" in "10ACB24-9P" means that it is a 2-ton unit (24,000 btu). I think that a rule of thumb is 300 cfm (cubic feet per minute) of air volume per ton so 728 cfm for 2 tons looks OK. You can ask the builder to check CFM with the new fan setting. You can also ask him to calculate your "superheat" which tells you how the evaporator coil is performing with regards to refrigerant charge.

My 3 speed Goodman air handler is set to low. I have all-metal ducts and a 2 story home so low speed works for me.

I had to re-read your post to see that it was the return grills that are vibrating and not the supply vents. Has the vibration stopped with the lower fan speed?

Best of luck to you.

Yes at the slower speed it does not vibrate. The blowers are on the brown wire, which according to the manual I have is one notch below full speed.

So slower air motion over the coil makes the air colder? So what would happen if I slowed it even more?

The contractor who started up the system in my new house did a test for the CFM and adjusted the blower speed accordingly. I suggest that you have this done by an expert.