You can't reduce it to 0 nicotine.
You can dilute it with either 0 nicotine eliquid or PG/VG and get it close to zero but that's it. If you add 5ml of VG to it, the combined 10ml will be at 1.5 mg/ml nicotine but it will change the PG/VG ratio and dilute the flavor. If you get the same eliquid in 0 nicotine and add 5 mls to the 5 mls you have, you'll end up with 10ml of 1.5 mg/ml nicotine eliquid and retain the flavor and PG/VG ratio.