Willito,
I can not offer a valid comparison of the Olympus LS-11 and the VN-8100PC as I have never seen or operated a VN-8100PC.
However after reading your post, I did go to the Olympus web site and looked at the specifications of the VN-8100PC and I have to admit, this recorder looks like it has the capability to work for recording nature sounds within limitations. Olympus states the frequency response range in MP3 high resolution mode (192Kbps) as being 70-19Khz and this would be sufficient based on my own experience.
However, in looking at the specifications and information, it would appear the VN-8100PC is a single channel mono recorder and I did not see any mention of internal microphone. Nor does it appear to use plug in SDHC card memory, it appears to have only 2Gb internal memory which, as highest resolution mode is MP3, a compressed file format, I would expect 2Gb memory to be very adequate.
In comparison, the Olympus LS-11 has three file formats available, WAV (uncompressed) as well as MP3 and WMA, both compressed formats. Whether this would be important to you would depend on what you plan to do with the sound files after recording them. In my case, I do a lot of post processing and sonogram analysis of my recordings and find there is a distinct benefit in the WAV format for lower noise when displaying sonograms. This is especially important when studying the harmonics of bird calls. It really boils down to what your use of the recordings you make will be.
Another plus of the LS-11 recorder vs VN-8100PC is that the LS-11 is a stereo, two track recorder and uses plug-in SDHC memory cards to augment the internal memory. I use 8Gb SDHC cards in my LS-11 and recorded files can be either downloaded over the USB cable (which works extremely well) or if you prefer, unplug the card and use a card reader on a computer. This option allows you the flexibility of not having to remove the recorder from the project, simply swap memory cards and the recorder can stay in use while you are processing sound files it has recorded. As I do a lot of overnight recording at remote locations, I find the SDHC card swap to be invaluable.
Another plus to the LS-11 recorder compared to what I read about the VN-8100PC, is that the LS-11 has an excellent pair of built in sensitive microphones(stereo), I have used the LS-11 internal microphones to make bird sound recordings of birds 7 meters to 15 meters or so away from my position as well as ambient field sound recordings and these recordings are full fidelity and of excellent quality. From what I read of the VN-8100PC, it does not have this capability but that may be as I could not find mention of internal microphone which I assume it must have of some sort.
I caution you, this is a comparison based on what I read of the VN-8100PC and of my own needs in nature recording which I find are filled quite adequately by the LS-11.
Now if you want to consider a step up in recorder capability above the LS-11 which is pretty basic, if you have a need or plans to do remote wildlife recording, I find the Marantz PMD661 to be the ideal PCM recorder and is what I use for overnight recording sessions where automated controls such as "pause on quiet" and "autotrack" can greatly reduce the amount of data when downloading and doing post processing. However, the PMD661 is a larger recorder, too big for a shirt pocket, if your need is for a shirt pocket recorder, the Olympus LS-11 is ideal.