Mild does not always mean lower nicotine then a regular. It usually means a milder or laughter blend (just like light ones are even more light) or the filter was different to allow more air in to make it feel lighter - milder. Unless the pack said 'lower nicotine and gave you a mgs then you essentially were a pack a day smoker and mild does not mean anything in reference to the nic strength.
Every some smoked different, inhaled more or less, let more urn up then someone else and so on. All of this effects the nicotine blood levels. Same with vaping, even a lower strength is you vape constantly will give you a higher level of nico in you body then if you vape and put it down for 30 mins.
Rule of thumb based not to rightly on the nicotine patch is q24mgs for a pack or more a day smoker 12-18 for one who smokes less or really did have a lowered nicotine ciggy. Lower nicotine ciggies have the nicotine strength on them so check that out. If it does thats your starting pint always starting lower rather then higher. If the low nic ciggy was a 1.7 then start with 12mg not 18.
I usually advice getting 2 strength like and 18 and a 24 for a pack a day smoker of reg nic cigs. You can adjust between the two or settle on one strength what ever works for you. Some people find that the higher nic level is perfect in the morning when nic level is lower, and during stressful time but evenings and relaxing its too much and they feel a bit greenish and switch to 18 (or 12 and 18 - whatever works).
I also advice that when you get your two strengths to start zeroing in on what's best for you you get them in the same flavor so flavor. is not confusing you as to what strength is better. Once you know the mgs that work best - the gor crazy with flavors.
Best advice I can give you. Sorry if its not the easy "just get this" answer but with vaping it never is. We all are different, smoked different have different experience so we have to find what feels best to use. Just remember with nicotine that if you start feeling queasy, a bit lightheaded or bussed (or just like you did with that first cigarette) its your body telling you your nic level is getting on the high side so you might want to switch back off a bit, use the lower strength or not vape as much for a while and inhale less deeply for a bit and so on.
ANd on the other hand if your vaping and still craving that cig in the middle of avape session, most likely you nic level is on the low side and you need to switch to the high strength nic, take more draws and/or inhale a bit more deeply.