Checking the various newspapers that have been uploaded online:
Feldon (under her maiden name of Barbara Hall) reached the $8,000 mark on the June 4th, 1957 program, won the $64,000 on the June 25th program, and appears to have been eliminated on the July 16th program (by that point in the run, contestants could win up to $256,000).
It is less clear when she started- the rules of the program online suggest she would have appeared on at least the May 28th program, but I am not sure if these rules were still applicable toward this part of the run.
Finally, as for your original question: given that Feldon appeared on at least two more episodes than March's vacation, it would seem that they would have had to share time- however, I haven't yet found a source stating when March's vacation began.