QProcess::execute(..) waits for the process to finish, that is why your GUI is freezing. Use QProcess::start(..) instead. To quit the process use the QProcess::close() function
Try this:
QProcess *myProcess = new QProcess(this);
myProcess->start("/ftest/vcm_test_2");
And when you want to close the process:
myProcess->close();
You can also connect your pushbutton's clicked signal to the process' kill slot:
connect(ui->pushButton_3, SIGNAL(clicked()), myProcess, SLOT(kill());