We can call vectors anything really, like vector \(x\), vector \(cat\), or even “that big vector”.
However, we normally use bold letters to write vectors, when we refer them as single letters.
For example, for speed we might write \(v\) while for velocity we might write \(\mathbf{v}\).
When we’re writing them on paper, it’s quite hard to write bold letters, so we either underline the letter or draw a small arrow above the letter, like \(\underline{\mathrm{v}}\) and \(\vec{v}\).
Note: I personally prefer writing \(\vec{v}\), as it’s standard, but apparently the cambridge examinations prefer bold, and hence the underline as well.