You can stop an optimization by clicking on the Stop Button at any time. When this happens, the optimizer will try to shut down cleanly, creating a runname.out file with the best parameter values found. It will also ask you if you want to make a simulation using these values.
You can start optimizing again from the old values by renaming runname.out as another file (such as runname.voc) and using that file as the optimization control file. You can also edit the renamed file and make any adjustments or additions you might want. Files with runname.out cannot be used as the input file. This is to prevent conflicts that might occur if Vensim were to try to read and write different information to the same file.