Please visit https://launchpad.net/~lubuntu-dev/+archive/ubuntu/lubuntu-daily. Here you'll find a collection of ppas
(Personal Package Archives) provided by Lubuntu's development team. If you need to know more about ppas, you can look at What are PPAs and how do I use them?. You may also want to know about how to remove a ppa: How can PPAs be removed?.
Now to the specifics ...
The first link I provided offers an Overview of published packages along with a filter: in your case, you'd filter for "Trusty". You'll see that 62 packages are available for Trusty. One of them is lxterminal
.
Let's assume you go ahead and install this ppa:
sudo add-apt-repository ppa:lubuntu-dev/lubuntu-daily
You'll then need to run
sudo apt-get update
to update your software sources.
At the next step,
sudo apt-get upgrade
all the existing software on your system will be upgraded to the version offered by the daily ppa, not just lxterminal
!
If you really want only lxterminal, you'll need to do a bit more work. You'll need to issue commands preventing upgrading of anything else. And that's covered here: Is it possible to only allow specific packages updates from a PPA.
By the way, since you've mentioned "vivid" in the title of your question, you can run a filter using vivid instead of trusty. As of today, both trusty and vivid have the same version of lxterminal, AFAICT.