Hey, if you're willing to do the "Tactics" chapter yourself, be my guest! It actually might work better that way: you could make up an NPC of your own -- an expert on doppelgangers, who'd told the VRF's narrator all about the strategies they employ -- and then write in-character as that person.
Basically, "Replacement" is simply how doppelgangers make a living: find someone, learn about them, kill them, pose as them. It's where you would describe their standard M.O. for replacing a person, and what they look for when they choose a target. Often, just one doppelganger will do this by itself; other times, a small group will replace a family, a party of adventurers, etc.
"Infiltration" is what I call it (just me BTW, feel free to swap these terms for something better if you like) when a group of doppelgangers set out to replace a
specific faction of people, as a means of achieving a larger objective. This is what the dopps in Paridon have done to the police force and government: not just assumed peoples' identities so they can live comfortably, but taken over a powerful institution so they can direct its activities from within. Often, this requires lower-ranked doppelgangers to assume roles that are pretty humble and demeaning (i.e. servants and toadies), the better to facilitate their superiors' access to victims of influence; getting stuck with such scut-work, in fact, is a big reason why Sodo'd wanted to take over his clan in the first place!
"Covert activities" is a catch-all for the nasty things that doppelgangers get up to -- extortion, sabotage, framing people they need out of the way, etc -- that don't actually require usurping an existing person's life. Between their disguise abilities (great for anonymous spying) and their mind-reading, doppelgangers are able to uncover so much dirty laundry on the people around them that they may not
need to replace you to manipulate you: what works for human blackmailers and so forth works for them too. Discrediting people who threaten them or the power their human cover-identities possess (e.g. by convincing others that their human
business rivals are doppelgangers!
) would also fall into this category.
As for "Psychological warfare", it's pretty self-explanatory. Compared to other monsters from the VRGs, doppelgangers aren't especially powerful; it's their ability to undermine group cohesion and morale which provides their
real scare-factor. They know that their best weapon is sowing doubt and suspicion among their enemies, so they do everything they can to set their foes at each others' throats, and render them unsure of their allies' identities, their
own identities, or even (via "gaslighting", a ploy tailor-made for dopps
) the very foundations of reality.
"Who [u]cares[/u] what the Dark Powers are? They're [i]bastards![/i] That's all I need to know of them." -- Crow
). 
