I'd like to hear thoughts about open D major (Vestapol) as a tuning for beginners, simply because fingering is relatively simple at first and new players can actually play stuff and concentrate on other skills like picking and chord progression.

I started out this way, partly because standard tuning turned the guitar into a medieval torture device that combined the concepts of boredom and impossibility for me. I then quickly progressed to open D minor (DADFAD, which oddly enough was the standard tuning for lute players in the baroque period), which allows easy fretting of both major and minor chords.  I very quickly learned a version of barre chords for all major/minors/7th chords, etc.  I also learned to recognize common chord progressions fairly quickly this way.  

From here it's an easy step to D Standard, DADGAD (Pierre Bensusan), and regular standard tuning.

The only downside I can see is that tabs are always written in standard and have to be translated.  There's also a wide stretch from string 5-6 when playing scales.  On the plus side, anyone into classical guitar can find lots of baroque lute music online with dadfad tablature ready to go.

I've heard it said that piano players and guitar players do things differently in their minds.
As someone with prior piano experience, DADFAD made perfect sense to me from the beginning--the piano keyboard just lights up on the (now linear) fretboard.  Maybe that's part of the reason why this clicked for me and why I still prefer DADFAD years after picking up the guitar.  I'm no longer using barre chords most of the time and have developed different moveable chord shapes similar to the standard caged system.  I can also run through scales as fast as I'd like so I see no reason to switch.

Tl;dr
I think standard tuning is stupid and kind of like a qwerty keyboard.  Furthermore, I think other tunings are better (for beginners).  Yes, I do know the ability to move across the fretboard in standard is partially given up as a result but I think it's a fair trade for a non-schizophrenic fretboard.

Is it okay to teach beginners unconventionally or should we stick with standard even though doing so makes people hate the guitar and stop playing entirely?

This is only my personal opinion and from someone that has very limited alt tuning experience. Since the majority of guitar
music is composed in standard tuning..and I would consider drop D to be in the standard tuning arena..I think new players should be introduced to it
first. Playing in standard tuning need not be anymore arduous than other tunings with a properly setup guitar running light string gauges.
Since I am old and set in my ways from 40+ years of playing I could be wrong here. I know that most finger style players use lots of alt tunings I even composed a song using EADBBE because it sounds so cool fiinger picking and I use a lot of double stops in my style of playing. YMMV.