If my thermostat was stuck open (P0128 code, low temperature needle), would I still have normal heat from the vents (I do)?

 

No, the engine would not develop enough heat and the vents would blow cold.
A properly working thermostat is needed for the cabin heater to work.
If you replace the thermostat then remember to look at old and new part carefully
Sometimes the new one or old may have a little index/burr that keys the thermostat
Also, beware not to over tighten nuts or bolts on housing.
They can crack and leak if over tightened
Last is always use a new gasket.