Hello Marco
It is possible to have different lead times for different vendors on MRP.
On the customizing transaction OPPQ, choose the option "external procurement" and make sure that the flag "scheduling: info rec agreement" is set.
With this setting, the planned delivery time is read from the info record or from the scheduling agreement.
After that, you should create a quota arrangement for both vendors (see note 1508647 for details).
On the quota arrangement, you can use the minimum and maximum size to define that the source will only be selected if the lot size is higher than 1KG or smaller than 1 kg.
If have a more complex logic to select the vendor, you can use BAdI MD_MODIFY_SOURCE.
BR
Caetano