I don't think you can exactly do what you are trying to do. As I think you've discovered, the DAQmx read VI is holding until the given amount of sample are read.
A possible work around if you know the number of samples to read and the sample rate is to generate the progress bar in a parallel loop. Have that loop begin when the DAQ mx read begins. Let's say you are doing 5000 samples as 50 samples /second., then you know it would take 100 seconds to read. Use timing functions in the parallel loop to update the progress bar as a percentage of time that has passed. If your timing is correct, the DAQmx read and the parallel loop should end at about the same time.