Ill tell you why your coughing when you first start vaping, its nothing to be concerned about its normal.
Completely switching to electronic cigarettes will be beneficial to your lungs. However, you must expect that at first, the lungs would need to purge out the tar, contaminates and mucus that have accumulated due to smoking.
This could take a month or even longer. Again, you might need to drink more water to assist in the process.