"J" is not a letter.

J is for... J!

J is not a letter - not in the Vulcan language, anyway.

A lot of people spell Vulcan words according to the Federation Standard pronunciation and alphabet. For instance:

T'hy'la instead of t'hai'la (the letter "y" is not used as a vowel in vulcan)
Eit'jae instead of eit'dzhae

X is not in the Vulcan alphabet either, but seems to have been used in words regardless. The correct spelling, for example, of "Xon" would be "Kson" when using Vulcan symbols.

The Vulcan alphabets, as taught by the Vulcan Language Institute:

Traditional Golic Vulcan:
S T P K R L A D O E V U H G Ch I N Z M Y F W B Sh Th Kh Zh Ts Dzh Ks

Modern Golic Vulcan:
S T P K R L A Sh O U D V Kh E H G Ch I N Zh M Y F Z Th W B