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As an IS only contributor I normally make all illustrations a vector and include a HiRES raster version. However, I have some illustrations that could have been vectors but I made them raster only, and they sell really well. I think you will need to experiment and over time you well be able to judge which version would sell better e.g. bloggers might lean towards buying only raster's and designers might lean towards vectors.

In the moment I'm also thinking to upload a vector-series as raster-files. Normally I sell as a nonexclusiv only eps-files. I never sold raster-files at istock.
Can I upload them as illustrations or only as photos (and have to start with three files) ?
I fear that this series of pictures will sell only as 5 credit-files. That's too cheap for me (at 16% commision).

At ss my raster-versions sell under 10 %. The vector versions are selling much better.

Eireann and redo are right. Because a subscription on SS costs over $200 per month, a high percentage of SS customers are art departments and professional designers, they prefer vectors. My vectors make more than 5x as much as the raster versions, which I only upload as an afterthought.

On DT, vectors do not get as many dls as the raster versions, but the non-subscription vectors cost a lot more per download than the rasters and so are well worth uploading.

It's worth it it doubles your portfolio.