I'm certain that it is more difficult to overdose when vaporizing (smoking) as opposed to snorting, or IV'ing, simply because of the rate of the amount being administered to one's body in a certain time frame. However, it is still certainly possible.
I think it's totally wrong, and almost harmful to actually say out loud that IV'ing meth would be safer than smoking it. Now, obviously, no meth usage is the best meth usage, no matter which ROA one chooses, but I have never seen so many people go from "doing allright" (smoking, snorting) to that one shot that some bonehead offers them - I could use a stopwatch...give it 3 months and they're either in jail, wanted by the police for robbery, completely bat shit paranoia crazy, or something worse. (Yes, I was making sweeping generalizations there, I know that not all meth IV users are any of the afformentioned, but the ones I know, are.) All I know is that I will NEVER EVER IV meth, or anything else for that matter, because I am sure I would fall in love with it, and end up talking to shoes in dark alleyways and talking to invisible gnomes. I'm sure it's fantastic, and if you use harm reduction tactics, hell, maybe it is better than smoking...but I sure as hell wouldn't recommend it to anyone.