How to insert a vector notation in Keynote/Pages?

Solution 1:

You can hunt for things like this in the [badly renamed] Emoji & Symbols [Character] viewer.
System Prefs > Keyboard > Keyboard & enable it in the Menu Bar; it will then be more easily accessible.

This type of mark is always going to be under 'Combining Diacriticals' though in this case it's even further down, in 'Combining Diacritical Marks for Symbols'

The trick to being able to see the whole list is to enable Unicode, which is less obvious than you'd think.
Top left there is a Settings button, which looks to be permanently greyed out. Click it, it works.

Unicode is way down the bottom, in Code Tables.
Once enabled, you can see it added to the list on the left.
Enabling it also then displays Unicode information under any selected character on the right.

You can also use the search, top right, for 'combining' which will find them.

In your case, the one you need is Unicode U+20D7
This, on it's own ⃗

Entered after any letter it will combine above, as in A⃗ B⃗ C⃗ etc

Entering it as a one-off is simply done by double clicking the symbol in the Characters pane.

If it's something you're going to be using frequently, I'd be inclined to add it to your text replacements in System Prefs > Keyboard > Text,

I just tested it as a replacement for _> so typing A_> will generate A⃗
Italics/Bold can be done before or after the fact, same as any other text.

A⃗

Solution 2:

If you're writing an equation, use Insert → Equation… (⌥⌘E). Here, you can insert LaTeX:

These equations can be display style or inline.