Your cardio is no doubt suffering from the introduction of nicotine into your system. Personally I think you're nuts to walk back into nicotine addiction after 3 years without but I guess if that's the only way you were gonna stay off the smokes it's fair do's
Incidentally nicotine introduced transdermally - like thru a nic patch - has a far lesser effect on cardio but the effect is still there and ofc doesn't sate the hand-->mouth aspect of the habit. You might have been better off starting out vaping with 0% nic to see if that worked tbh, after 3 years off the smokes you couldn't be having a real physical craving for nic and the hand--->mouth thing may well have been enough to get you thru the urge to smoke.