Is there any risk in vaccinating just sows against PRRS but not piglets? Why using type 1 vs type 2?

A user from Vietnam asked the following question through the section "Updates on PRRS" of PRRSControl site:

"In Vietnam, many farms vaccinate sows against PRRS but not piglets. Is there any risk to these farms? And, is there any risk if farms use type 1 PRRS live vaccine for sows and type 2 PRRS live vaccine for piglets or the opposite?"

See Enric Marco's answer below...