Мне вполне удобно.

Because Russian often uses the dative case to show the person who experiences a state or condition.

So Мне вполне удобно is literally something like To me, it is quite comfortable/convenient.

This pattern is very common in Russian:

• Мне интересно — It’s interesting to me / I’m interested
• Мне трудно — It’s hard for me
• Мне холодно — I’m cold
• Мне удобно — It’s comfortable/convenient for me

So я is not used here, because the sentence is not built as I am X, but more like for me, it is X.

Here удобно is a predicative word (often taught as an adverb-like form), used in an impersonal sentence.

It comes from the adjective удобный:

• удобный стул — a comfortable chair
• удобное время — a convenient time

But in Мне вполне удобно, it does not describe a noun directly. Instead, it describes the situation as experienced by someone.

That is why it stays in the form удобно and does not change for gender or number.

In a sense, yes—but Russian does not need to say it.

English usually requires a dummy subject:

• It is comfortable
• It is convenient

Russian often leaves that out:

• Мне удобно
• Мне вполне удобно

So the sentence is already complete in Russian. There is no need to add a word meaning it.

If you want, you can make the context explicit:

• Мне вполне удобно здесь. — I’m quite comfortable here.
• Мне вполне удобно сидеть так. — It’s quite comfortable for me to sit like this.

Вполне often means quite, perfectly, fully, or entirely, depending on context.

In this sentence, it usually means something like:

• quite
• perfectly
• comfortable enough / convenient enough

So Мне вполне удобно often has the feeling of:

• I’m quite comfortable
• It’s perfectly convenient for me
• This works fine for me

A useful nuance: вполне often suggests sufficiency—not necessarily dramatic emphasis, but this is absolutely acceptable / good enough.

For example, if someone asks whether you need a better chair, you might say:

• Нет, мне вполне удобно. — No, I’m perfectly comfortable.

It can mean either one, depending on the situation.

• If you are talking about a chair, clothes, posture, etc., it usually means comfortable.
• If you are talking about a time, place, arrangement, or plan, it usually means convenient.

So Мне вполне удобно could mean:

• I’m quite comfortable
  or
• It’s quite convenient for me

Russian uses the same word for both ideas much more often than English does.

No.

• Я удобно is ungrammatical.
• Я удобный does not mean I am comfortable.

Удобный describes something as comfortable, convenient, or handy:

• удобный диван — a comfortable sofa
• удобное приложение — a convenient/user-friendly app

If you say Я удобный, it would sound like you are describing yourself as a useful or convenient object/person, which is usually not what you want.

To say that you feel comfortable or that something is convenient for you, use:

• Мне удобно
• Мне вполне удобно

Yes, Russian word order is flexible, but Мне вполне удобно is the most neutral and natural order.

Why this order works well:

• мне sets up for me
• вполне modifies удобно
• удобно comes at the end as the main idea

Other orders are possible, but they sound more marked and usually add emphasis or contrast.

For example:

• Мне удобно. — neutral
• Мне вполне удобно. — neutral, with quite/perfectly
• Удобно мне. — contrastive, like I’m the one who’s comfortable
• Вполне удобно мне. — possible, but much less natural for a beginner

As a learner, it is safest to keep Мне вполне удобно.

To make it a question, you usually just use intonation:

• Тебе вполне удобно? — Are you quite comfortable? / Is this convenient for you?

To negate it, there are two common possibilities:

• Мне не вполне удобно. — I’m not quite comfortable / It’s not entirely convenient for me
• Мне неудобно. — I’m uncomfortable / It’s inconvenient for me

These are not exactly the same.

• не вполне удобно = softer, partial negation
• неудобно = stronger, more direct

Also, неудобно can sometimes mean awkward or embarrassing, depending on context:

• Мне неудобно спрашивать. — I feel awkward asking.

So in your sentence, the positive удобно is about comfort or convenience, but the negative form can develop extra meanings.

Yes, very natural.

Мне вполне удобно is a normal, polite way to reassure someone that the current arrangement is fine for you.

For example:

• someone offers another chair
• someone asks if you want to move
• someone checks whether the time or place works for you

In those situations, Мне вполне удобно sounds calm and polite—often a little more reassuring than just Мне удобно.

So it is a useful phrase to remember as a ready-made response.