nornen:Is it common to use the accusative of direction with personal pronouns (or humans in general)?

It's not only uncommon, it is virtually absent so that this can be called erroneous. The accusative of direction (!= adverb of direction) appears only with typical names for places, mostly cities.

It is erroneous to use the accusative of direction with anything that is not a location. You could theoretically use it with a personal pronoun only if you consider a person to be a location.

For example, if you wanted to say "I am going to work", you couldn't say "Mi iras laboren". Laboro, work, is an activity, not a place. You could, however, say, "Mi iras laborejen" (I am going to the workplace).

erinja:It is erroneous to use the accusative of direction with anything that is not a location. You could theoretically use it with a personal pronoun only if you consider a person to be a location.

For example, if you wanted to say "I am going to work", you couldn't say "Mi iras laboren". Laboro, work, is an activity, not a place. You could, however, say, "Mi iras laborejen" (I am going to the workplace).

I think Kirilo81 was talking about the accusative of direction and not the adverb of direction.

It's the same idea, in this case. A directional accusative only works with a location (even a metaphorical location, sometimes) as the "destination" of the movement.

You can imagine, using my same example, that "Mi iras laboron" makes no sense whatsoever, whereas "Mi iras laborejon" is a rare but correct choice.

An expression like "iri vin" wouldn't work, in my opinion.

PMEG page on the accusative of diection

erinja:It is erroneous to use the accusative of direction with anything that is not a location. You could theoretically use it with a personal pronoun only if you consider a person to be a location.

What's your source for this? Is this kontraŭ fundamento?

The accusative of direction seems to be used in the ekzercaro for things that I don't consider to be locations.

fundamento ekzercaro:
Mi metis la manon sur la tablon.

I put my hand onto the table. I don't consider a table to be a location.

Zamenhof also seems to use pronouns as sort of a short hand for the location where a person happens to be. Russian does something like this, perhaps Polish does as well.

fundamento ekzercaro:
Je la lasta fojo mi vidas lin ĉe vi.

ĉe vi; at you. Perhaps Zamenhof did consider people to be locations...

And then this is how Ivy Kellerman describes the accusative of direction in her Complete Grammar of Esperanto.

Kellerman:
46. When the verb in a sentence expresses motion, the word indicating the place, person or thing toward which the motion is directed is given the accusative ending. This is also true if the word is the complement of any preposition which does not itself sufficiently indicate motion in a certain direction. (The prepositions al, to, toward, ĝis, as far as, tra, through, express motion in the direction of their complements, and could not well be used except in a sentence whose verb expresses motion. Consequently the accusative is not used after any of these three)

*emphasis mine.

mfar:I'm assuming you changed "kiom" to "kiel" because "kiom" only refers to quantity?

Yes - and kiel can be used for degree as well as manner. As PMEG puts it: en kiu maniero aŭ grado.

Alkanadi:Is it better to change vi to oni?

It depends on how emphatic you wish to be. I remember a language textbook in which the author put in italics:
Never mind slow progress, so long as you keep going!

After thinking about this, I remembered that I actually have seen the accusative of direction used with a pronoun several times in the ekzercaro of the fundamento in the story of La Feino (the fairy).

Zamenhof:
...tio ĉi estis feino, kiu prenis sur sin la formon de malriĉa vilaĝa virino...

"...This was a fairy, who took onto herself the form of a poor village woman..."

Here, "formon" (form) is the proper accusative, the direct object, of prenis (took). Si (the reflexive third-person pronoun, herself, himself, etc.) takes the accusative form to show the direction of the action.

This is one of several examples.

Alkanadi:
Is it better to change vi to oni?

I don't think so. Using vi (you) makes it more personal. You're giving advice directly to another person.

This has a different feel from using oni (one). Oni could apply to anyone, so it feels more impersonal and less direct.

eshapard:After thinking about this, I remembered that I actually have seen the accusative of direction used with a pronoun several times in the ekzercaro of the fundamento in the story of La Feino (the fairy).

Zamenhof:
...tio ĉi estis feino, kiu prenis sur sin la formon de malriĉa vilaĝa virino...

"...This was a fairy, who took onto herself the form of a poor village woman..."

Here, "formon" (form) is the proper accusative, the direct object, of prenis (took). Si (the reflexive third-person pronoun, herself, himself, etc.) takes the accusative form to show the direction of the action.

This is one of several examples.

I think nobody is doubting that a preposition can assign the accusative case to its dependent phrase.
You original example was "Mi volas malrapide vin iri. I want to go [to] you slowly."
"Vin" without a preposition denoting direction.
?Mi iras onklon. ?Mi ĵetis la ŝtonon la amikon. ?Iru min!