The gemara tells of two stories where R. Shimon (son of Rebbi) was sitting by his father and a contract was presented to them. In the first story, Rebbi was unhappy with the way the contract was written. R. Shimon realized that Rebbi thought that he wrote it, so he responded that R. Yehuda wrote it. The gemara considers this actual loshon ho'rah. The Rashbam explains - לא היה לך להטיל אשמה עליו, היה לך לומר איני כתבתיו. This implies that since R. Shimon had some other way of removing the guilt from himself, it is loshon horah to convict someone else. But had R. Shimon not been able to convince his father that it wasn't he who wrote it, unless he were to mention the name of someone else, he would be allowed to do so. The rationale would be that one is entitled to protect his reputation, even at the expense of loshon horah on another, if he has no other way to convince others of his innocence.
In the second story, Rebbi was impressed with the writing of the contract. R. Shimon saw that his father thought he had written it, to which he responded that R. Yehuda wrote it. The gemara considers this not to be actual loshon hora, but to be avak loshon horah. Why? The gemara explains - שמתוך טובתו בא לידי רעתו. The Rashbam explains that לידי רעתו means לידי גנותו. By R. Shimon praising R. Yehuda he would also come to mention the גנות in him - מתוך שמרבין בשבחו מזכירין שם גנות שבו. We can make a subtle diyuk from the Rashbam that he seems to understand that the issur of avak loshon hora is that the speaker will be led to say actual loshon horah. From the Rashbam we don't necessarily see that if by telling something positive it will lead someone else to say something negative, that the positive statement would also qualify as avak loshon horah. However, the Rambam (Dei'os 7:4) clearly writes that the issur of avak loshon hora is that it will lead others to tell loshon horah. Even if the speaker will not be led by his words to tell of his faults, it may lead other to do so (see chofetz chaim, be'er mayim chaim 9:1, who explains that opinion of the Rambam to be an issur of leading others to loshon horah). Perhaps the nafka mina would be if one would publish praises of someone knowing that it is likely to cause the readers to point out his faults, according to the Rambam the writer would be in violation of avak loshon hora, but according to the Rashbam it would be permitted.
The chofetz chaim (hagaha 9:1) asks, why isn't every case of avak loshon hora an issur of lifnei iver since it is a stumbling block that leads others to doing an issur? He explains that lifnei iver may only apply when it is very likely to lead to the person doing an issur, but avak loshon hora would apply even if it is unlikely that the listeners would be led to speak badly about the person, since it is at least remotely possible. However, the chofetz chaim is bothered that the language of the Ramabam and Rabbeinu Yona implies that even if you directly lead someone to speaking loshon hora it would only constitute a violation of avak loshon hora, not lifnei iver? The chofetz chaim explains that the answer CANNOT be that lifnei iver only applies when the person couldn't have done it without you, but here he could have spoken loshon horah without you, but would not have. This answer doesn't hold because any time the person would not have done an aveira without you, even though they could have, would still qualify as תרי עברי דנהרא and be a d'oraysa issur of lifnei iver (see here). The Chofetz Chaim answers that in a situation where it is likely that they would have spoken bad about the person even without you speaking positive about him, nevertheless, since the positive talk may lead them to speak badly, it qualifies as avak loshon horah.
It seems that the question of the chofetz chaim only applies according to the Rambam and Rabbeinu Yona's definition of avak loshon hora, that it will cause others to speak badly. But, according to Rashbam that it is an issue of the speaker himself moving from positive to negative speech, it has nothing at all to do with lifnei iver.