Finally posting after referencing these posts for about a year, and thought it was time to dump some experience/observations in here to help others.
I started using the Vaporesso tanks back early in the summer, also diluting shatter with Wax Liquidizer (I've tried Holy Terps and some others too, came back to Wax Liquidizer for now).
So far I've extensively used the Guardian and both Veco's - the 2ml and 4ml plus. I have found that the Veco hands down worked better than the Guardian in terms of quality of hit and duration of coil. The primary advantages of the Guardian - leak proof, more damage resistant - have not been issues with my Veco either, so I have completely stopped using the Guardian with no regrets.
I also found that the 4ml Veco Plus works much better than the 2ml version - larger hits, richer hits, longer duration of coil, and fewer nasty/burnt hits. If I had to speculate, I'd say the larger chamber and wider gaps between the middle chimney and outer glass allow for a more favorable flow of concentrate into the coil. If you've played with this stuff enough, you've witnessed your concentrate sort of magnetically sticking in those smaller gaps even though its completely liquid. It's sort of like a suction effect that the 4ml tank is less prone to.
I'm using a temp control mod - the VooPoo Drag. I tweak my settings a lot - usually the TC setting I run is 1400, but I've rolled it back to about 1380 recently. Temperate setting generally sits at 425F, but I roll around 420-435 often depending on temperate/the coil, etc. It's a lot of feel at this point. Wattage sat on 33W for months (it never reaches this in TC mode anyway), but I've rolled back to 25W lately because I've recently been killing coils much quicker.
Speaking of Coils, I used the .5ohm ceramic EUCs. I found these had larger holes than the .3s, which is why I've rolled with them. I haven't ever seen one read at .5, however. In fact, over the months they've gotten lower-and-lower, recently usually around the .430-.445 range. I'm just chalking this up to shoddiness in the manufacturing process?
In terms of prepping/priming coils, I've begun the practice of constantly having my next coil soaking in a small vial of concentrate. I used to go maybe 4-5 days between coils, but recently its been about every two days (I do have a lot of friends using it in social situations, so amateur hands likely help burn them out faster). I understand this is a luxury, since I generally have a good supply and thus spare product to just let these soak in, and not everyone could practice this.
Now all that being said, while i feel like the Veco Plus 4ml is the best thing I've come across, I'm not 100% satisfied - I'm still looking for that better tank.
I've noticed the new Cascade tank on their site - I'm back and forth about trying. That quad-coil looks like it could do a better job of soaking up concentrate for nice rich hits. Also, I find the Veco burns coils out from the top, and my hunch is that the concentrate is coming from beneath that point, so it's not always making the climb up against gravity. The cascade seems like it would eliminate that issue since it's all spilling down from above the coils? But those tiny holes are sort of concerning. Also I know people don't like bottom airflow due to leaks - I've had mixed experiences there. Anyone tried it?
My concern with ceramic vs. Cotton wicks is that the cotton will get nasty faster than the ceramic and will need changing more often. This is soley based off using ceramic coils for ejuice after using cotton coils for ejuice for a long time. The flavor just lasts longer in terms of flavor. How long are the Smok coils lasting you? In the ejuice world they are not known for there longevity.