I'm always skeptical of claims by the "dietary supplement" industry, since they're not held accountable to the FDA and their "studies" are usually nothing more than anecdotal with no controls to check for placebo effects - and this seems especially true for vitamin E. It's also hard to imagine how topically-applied vitamin E oil could penetrate all the way to the cavernosa, the chambers that fill with blood during an erection. And you don't need circulation in the veins, the blood enters via arteries and the veins are blocked - that's what keeps the blood trapped and your erection hard (that's how cock rings work).
That being said, "jelqing" or "milking" a semi-hard cock (the veins are still partially open) does cause circulation of blood and, by increasing the pressure in the cavernosa can cause the micro-tearing of its tissue - which then heal slightly larger than before (same idea for increasing the size of muscles through exercise). This milking also stretches the suspensory ligaments that affect length - except that the micro-tearing is more difficult and takes more time because ligaments are very tough tissue.
Personally, I "milk" my semi-hard cock in the shower once in a while but, primarily I depend on pumping. Unfortunately, I didn't discover pumping until my late forties (via a friend whose doctor prescribed one after a bicycle accident that injured his groin). I also liked how it feels. I've been pumping fairly regularly, aiming for a 15-minute session at least 3 days per week. Over the 25 years of pumping, I've gained about 2 inches in length and a bit over 2 inches in girth - permanent gains. I started with a pretty average size of 5.5 x 4.5 and I'm now at about 7.5 x 6.5. Of course, immediately after a pumping session, the girth will be even larger because of temporary edema - but that goes away in 6 or 8 hours. Gains from pumping - or any regimen for that matter - takes a lot of time. It's not like exercising to develop killer biceps - muscles respond much quicker than cavernosa tissue or ligaments. I only wish I'd started pumping when I was younger. Note that pumping is FDA approved as an ED treatment. For ED, the cock is pumped up for 5 or 10 minutes - then an elastic cock ring is applied at the base of the cock to trap the blood keeping it hard. The FDA even has a website with instructions and cautions (like leaving the ring on for more than 30 minutes). Pumping to increase size is something to start gradually. At first, there's some pain as the skin is stretched tight when pumped up. So both the time and "pressure" (actually vacuum) is increased over a period of weeks as your skin also grows to adapt. But I enjoy the feeling of pumping, love looking down at my larger than ever tool, and the look of my "freshly pumped" cock in jeans and the looks my bulge gets in public. All good for the ego and in overcoming that "inadequate" feeling I always had in the high-school locker room. Of course, some guys are lucky to be "show-ers" (nearly as big soft as hard) while others, like me, are "grow-ers" looking very small when soft. For me, as a gay "top" man, size is definitely a big deal.
Anyway, that's my experience and considered opinions. I hope it helps you decide what direction to take.