“What could I do but bow acceptance? It was Mr. Hawkin’s interest; not mine, and I had to think of him, not myself; and besides, while Count Dracula was speaking, there was that in his eyes and in his bearing which made me remember that I was a prisoner, and that if I wished it I could have no choice. The Count saw his victory in my bow, and his mastery in the trouble of my face, for he began at once to use them, but in his own smooth, resistless way”

I'm having a bit of trouble understanding what he means by "he began at once to use them, but in his own smooth, resistless way." As in, using his victory and mastery upon Jonathan?

Also oof, Jonathan is so aware that from Dracula's body language alone, he is the captor, and that any choices he's giving Jonathan are smoke and mirrors..m

Yes. So, basically, Dracula can see from Jonathan's reaction that he has won this interaction. Jonathan has read the room (it says DANGER DANGER DANGER) and isn't going to break this delicate balance where neither of them admit to this being a hostage situation. He's playing along, because he correctly reads that refusal will mean a worse situation for him. At least if they're still playing pretend that this is a friendly interaction, Dracula won't be outright locking Jonathan in a dungeon or physically hurting him or whatever else (vampire stuff, not that Jonathan knows that for sure here). So Jonathan bows acceptance. He 'willingly agrees' to stay another month, and tacitly accepts the lie that Dracula has been an excellent host/employer who deserves his utmost efforts in whatever way he wishes, because it's his job to answer those wishes. After all, he can't disappoint Mr. Hawkins (yet another form of power Dracula holds over Jonathan, even outside all supernatural/physical power - he could potentially hurt his career and maybe even that of his boss).

So yeah. Dracula sees Jonathan agree to all this. And he knows he's won. Perhaps Jonathan's silent bow even indicates to him how bitter a pill it is for Jonathan to swallow, that he doesn't even want to say it aloud. So Dracula, loving to make things worse, immediately starts to use his victory to ask for more. Specifically, by being "smooth, resistless", he's putting on a super charming front. His suavest voice, his most charming smile. He's acting super nice while he makes Jonathan actively lie about his safety and condition. He implements his mastery of the situation to force Jonathan to go along with him in a way that not only will serve Dracula by removing potential suspicion from others, but is also designed to make Jonathan feel complicit. (He also gave Jonathan three envelopes, but Jonathan only wrote two letters. This may suggest to Dracula that his guest has few people who would miss him, or at least who he feels any need to explain his absence to. Additional info.)

Not to spoil any specifics, but... this is a pattern. Both the "being extra charming in the worst moments" and the "making people feel complicit" stuff.

(If you want, I also wrote a meta last year comparing some of Dracula's wordplay in this scene with a later scene on September 18th, but it's obviously got spoilers. )