It will work fine.
Anything that has a USB port on it will, 99% of the time, have an output voltage of 5v, because that's what a computer's USB port is, and the whole idea of having a USB converter is to match a computer's USB port.
In addition, if the port is capable of a 1500mAh output (1.5amps) that doesn't mean it's going to be pumping out 1.5a all the time. It simply means that's the max output of that port, it's like a speed limit. If the street sign says 55, that means you can't go over 55mph, but you can still go slower (although you go too slow and you'll still get a ticket, the analogy is beginning to fall apart heh heh)
The circuit in the charger causes the charger itself to put out 420mA. It's not going to draw anymore current than it needs, it will draw the same current on a port with a 500mA output, or a 3000mA output.
No need to worry, charge, and vape away!