how to say i miss you in vietnamese?

Answer 1

Meh...I'll be the expert *lags...so tired*

Yes, as we've established, Vietnamese is quite formal in speech. You address others by their title.
Also note that you should try to avoid addressing yourself and the person by their names as it is formal speech. Common speech is no different. Address by TITLE ONLY.

Tôi - I (neutral)
Con - Child (literally I for when talking to adults)
Bạn - friend (literally)
Ông - Old man
Bà - Old lady (usually married)
Cô - Young looking lady [female teacher]
Thầy - male teacher
Bác - Someone (male or female) older than your parents (by abit)
Chú - Some man younger than your parents
Mợ - Your mother's brother's wife
Cẫu - Your uncle or a sweet looking boy =)
Dì - mother's younger sister ONLY
Vượng - Your parent's sister's husband
Thiếm - father's younger brother's wife ONLY
Cha (Ba) - Father
Mẹ (Má) - Mother
Anh - Older brother or older male
Chị - Older sister or older female
Em - Younger sibling or younger person than you
Mày - infromally, 'you' -do NOT use when speaking to elders lol. *slapped on the head*

To avoid all this confusion just say "nhớ quá à" where à is just to add depth to the phrase. Literally meaning "I missed you"

nhớ means to miss.

So use any of the titles above and fill in this slot

(your title-who are you in relation to the other person) nhớ (who are they to you)

eg.
Anh nhớ Em (I missed you - Male to female).

quá means lots. Lắm also means the same thing, lots.
or even rất nhiều meaning alot.

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I stress my concern as to the difficulty of what this may be. Sorry if you are confused but it's the only way to know lol.

If you would like to make it more vibrant:

I missed __ very very much
(title) quá là nhớ (other person title)
quá là expresses the great sense of things. (greatly)

Answer 2

I've cribbed this from another forum; but it may help!

The phrase you want to use is "Tôi nhớ chị." Literally translated, it
means "I remember/miss older sister." Chị is a respectful term for a
female slightly older than oneself. You can also emphasize this by
saying "Tôi nhớ chị lắm." [I miss you very much]. The use of the
pronoun pair tôi/chị is appropriate for most cases, but can seem a bit
aloof for younger people. If you and her are in your 20s, you should
use "[Your name] nhớ [her name]" because it sounds more personal.

Best of luck.