Vishay is a much larger company than LiteOn, and with that increased size comes some advantages compared to smaller companies.
However, LiteOn has been producing a variety of products for nearly 50 years. Their datasheets are accurate, and their products are good quality. We have been selling LTR-329ALS-01 for 4 years with no major issues.
I do not think you will have any issues using this product in your application.
Well the price for the part LTR-329ALS-01 is much lower than similar products for example VEML3328SL which is from Vishay produced at large quantities.
Also I am not sure if the spectral response provided in the datasheet is based on simulation or an actual measurement. It has a very nice and smooth curve. Typically a light sensor has some oscillation on its spectral response and does not have any any sharp edges on the response. See below:
The price difference between the two products mentioned can be explained by noting that the Vishay product offers greater functionality, in terms of providing data for 5 different spectral responses versus two for the Lite-On product.
As for the representation of these spectra, it should be noted that both are labeled “Typical Performance Characteristics.” This means that the information is not guaranteed, and does not come with any implied precision or resolution. It is not uncommon for information of this kind to be presented in “smoothed” form, to provide a neater chart and prevent attempts to use the information is ways that is not intended or appropriate.
As for the slopes in the various response curves, compare those above with that shown at the left side of the red channel for the Vishay device.