When converting your pencil drawings to vector, you will probably lose some details. But you can always tweak then back by using the Pen Tool. This method below is to save you time if you have a lot of pencil drawings to convert into vectors. I strongly suggest you try with one drawing first and then do the other ones if the method works fine.
You can do a lot by simply scanning your images. It's not necessary to redraw them with your tablet or the pen tool, unless the originals are very small. But even then, you can do the steps below and then tweak them.
If you don't like watching video tutorials (I don't), here's a technique you can read. It works very well for black and white illustrations, and pencil drawings.
This is the way to get the quickest results and still keep a lot of your drawings' details:
1) Scan our drawing at the highest resolution your scanner can do.
Usually 300-600 ppi are alright if the drawings are already at big size. Scan them in grayscale mode. Don't use more than 600dpi unless you tested the result with ONE drawing first; most of the small office scanners don't really scan at 600-1200ppi even though they offer the option and can create fuzzy results.
2) Open your scanned images in Adobe Photoshop and "clean up" your drawings.
An easy way to adjust quickly the contrast of you drawing is to use the "Levels" (in the menu "Image/adjustment").
To make sure the black is a real black, click on the black color picker, and click on the darkest area of your drawing. Then adjust the white or remove the very light gray that could be on the drawing by using the white color picker, and use it on the area you want to see as pure white. As a side note, if the white doesn't become pure white, double click on the white color picker in the "Level" window, and change the values of CMYK to 0 everywhere.
3) Once this is done, lower the resolution to 100ppi.
You can lower it to 72dpi if you want, the important is to have a big image for the next step.
Make sure your width, length and resolution are linked!
4) Copy and paste that image in Illustrator.
It should be very big in Illustrator, and the biggest it will be, the more details you will keep in your drawings. That's why it's good to scan at high resolution and then adjust the resolution in Photoshop: it will increase the dimension.
5) In Adobe Illustrator, select your image and click the "Image Trace" button in the menu.
To see your Image Trace options, click on the small "Image Trace Panel" next to the word "View" on the top menu.
You should select "High Fidelity" in order to keep the black and gray details from your pencil drawings.
You can adjust the settings as you wish but for pencil drawings, the high fidelity option in color is the one that gives the best results quickly. You will see a preview of the vector image if you click "preview" on the option panel.
6) Once you're happy with the result, you can click the button "Expand" on the top menu.
Now, you have a big vector image that you can resize to a normal size.
You might need to also delete the white in your new vector; you can do this by selecting one white area, and then go in the menu "select/same fill and stroke", and delete this.
You can now tweak your design easily with the pen tool if necessary.
Here is the result:
For this example, the image was "small", so you do lose details.
But with a scan at bigger size, your paths in Illustrator will be more precise and have more corners too; the image will retain more details as well.