Unlike AI, AE alsways works with pixels. AE is designed for video and there are no vectors in video.
But AE is capable of working with vectors, which means you can scale a vector file as large as you want and it sill stays sharp and crisp. You have to activate continous rasterization on all vector layers and comps in order to do so.
The preview of AE always shows pixels, because AE is a video tool. You can zoom in the preview to 1600% and the pixels will be gigantic. Sometimes this is necassary for detailed work. However, everthing in the preview is rasterized and you cannot retain quality when zooming into a rasterized image.
Don't mistook those two concepts: scaling a vector is no problem, if CR is activated - zooming into the preview always shows a pixelated image, even on vectors.
If it is a vector based Illustrator file you can click on the Continuous Rasterization button. If it is a raster image from Illustrator then you will only have pixels as there are no vectors contained in the image.