Hello there,
I checked my VueScan parameters so many times against recommendations… But whatever I do :
Do you have any clues about how to fix that problem ?
Thank you for reading me 
Where are you seeing that the DPI on the DNG is 240? I believe the DNG will have an embedded preview in it, so whatever you are using to measure the dpi could be looking at that preview.
DPI is also a measure that is more specific to printing… as you are editing the negative, the resolution (dimensions) are probably a better measure to be looking at. Are the dimensions the same between the TIF and the DNG?
Dimensions are exactly the same whenever I use TIF of DNG yet the TIF file shows correct DPI value.
I see 240DPI whenever I open the DNG in photoshop camera RAW or if I export it from Lightroom.
I juste spoke to a friend who have the same workflow, she happens to have exactly the same “issue”.
Raw files have no inherent DPI. They just have total pixel dimensions.
ACR assigned a default DPI of 240 (I think now it’s actually 300) so Photoshop will know the image’s DPI (Photoshop has some simple resize & print functions that rely on an image having a DPI). You can change an image’s DPI in ACR by clicking where the DPI is displayed at the bottom of the ACR window. You can also change the color space & bit depth. Any changes you make to DPI, color space, & bit depth that are eventually fully opened in Photoshop will be used for future images opened in ACR.
Similarly, Lightroom’s default export DPI is 240, but it’s an arbitrary default that does not effect the final image. You can change it to whatever you want in the Export options.
But honestly, in the end DPI does not matter unless you are printing, and changing an image’s DPI does not change the image’s pixel data; it just specifies how many pixels are printed per inch.