Synfig is the most suitable for doing vector animation, and you can almost animate mostly everything which is a vector graphics.
You can animate the modification of shape, color, outline, width, transformations such as rotation, scaling, skewing, transparency etc. If your primary focus is on just performing animation on vector shapes, Synfig has all the capabilities to do so.
As already mentioned Enve is also a good choice 
and seems to have similar capabilities to Synfig, (maybe a little better in some cases). Since I haven’t used Enve I don’t have much to say. But Synfig has existed longer than Enve and there is active development, Synfig is pretty stable to use.
I would also like to point some drawbacks of Synfig to consider:
If most of your animations include Typography animations, Synfig might not be a good choice, since there is very little support to animate text. You can include text in your animations but, creating effects for per character/word in a text is not directly possible.
Of course you want to use it for vector animation, but also needed to mention that After Effects has capabilities for video manipulation which Synfig doesn’t.
Overall through my experience, Synfig is actually a good tool for animating vector graphics overall, compared to After Effects sometimes you might need to do a little extra work, but you can do it 
.