Yes, an atomiser spray pump can deliver metered doses. But, this is a medicine and not designed as a leisure product to make anyone happy with its performance. You use it as little as possible and are glad to throw it away when done. It's also more expensive than
vaping because to ensure metered doses the whole manufacturing chain and testing procedure are more costly. Imagine sending out a medical inhaler that delivered double the dose, or one-tenth of the dose - that might be extremely problematic and expensive for the company concerned. But in an ecig the 'dose' varies by at least a factor of 10 across all the devices available; it varies perhaps by a factor of 3 or 4 across examples of the same product; even in the same device it will vary by at least a factor of 2 according to usage parameters. It is not important because it's like coffee, if you need more you drink more; if you somehow get given a cup that is mega-strong then you pretty much know in a few seconds.
Maybe an ultrasonic device or a piezo-electric device could also be constructed that could deliver a metered dose of nicotine aerosol. So there are going to be different routes to achieve this.
But these solutions are not
vaping. The reason why we can freely use the term 'e-cigarette' for what is far more accurately described as an electronic (or electric) vapouriser or EV is because the vapour is specifically engineered to replicate cigarette smoke. It is a vapouriser that, in the most important respects, replicates cigarette smoke. It's done the way it is because that is the best way to replicate cigarette smoke that we know - not for some irrelevant reason such as metered dosage.
There are all kinds of fairly cheap ways of producing a nicotine-containing aerosol but they won't be vaping as we know it. It will be strictly a personal vapouriser and may look and feel like an asthma inhaler. That's how you get a metered dose.