How can I put vst plugins like Sylenth1 and Massive in Logic Pro X ?!

It will come with an installer, and install it in the operating system. And you want AU plugins, not VAST, although I believe LPX will recognize VAST plugins. Once that installer that came with the plugins is run, that's all there is to it.

Then inside LPX, create a new software instrument track. From the instrument menu in the track info, select what you want to assign it.

I suspect you meant VST rather than VAST..... and Logic does not support VSTs

Logic Pro X only works with 64bit AU (AudioUnit) plugins so you will need to obtain those..

Massive (the current version) is available as a 64bit AU Plugin

Update via NI's website if you are using/still have the older 32bit version

Sylenth is now available as a beta 64Bit AU plugin..

https://www.lennardigital.com/home/

Also there are some AU plug ins that are able to host VST plug ins. Doing a quick google search brought up a few, but I use Maschine to host VST's within Logic.

Just to add...

Even though this may be obvious, the VST plugins that can be wrapped have to be written for the Apple OSX Mac operating system, Windows .DLL plugins cannot be used.